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It is frequently suggested by persons, presumably familiar
with previous work, that it would be desirable to include gravity
observations on the Carnegie, We are told that ocean gravity
work, of requisite accuracy, is one of the few remaining heroic
problems. JBefore deciding on any additional work a careful
survey is made of existing knowledge and of the actual require-
ments for obtaining trustworthy data. I was thus led into an
examination of past ocean gravity work, viz., that by Dr.
Hecker on three cruises between 1901-09, under the auspices
of the International Geodetic Association and published in
three monographs aggregating 500 quarto pages. Examina-
tion was made at first only in general ; however, I soon became
engaged in an exhaustive examination of the entire problem,
not only going this time more deeply into Hecker's methods of
observation and of computation, but also consulting experts in
thermometry and barometry of the U. 8. Bureau of Standards
and of the LI . S. Weather Bureau, besides well-known geod-
esists and physicists. The final result of this preparatory study
was the paper " On Gravity Determinations at Sea" published
in this Journal, January, 1911. This paper receivea the en-
dorsement of several well-known investigators to whom the
manuscript was submitted before publication. Its special aim
was to arouse general interest in this difficult subject and to
assist in making clear the direction in which further advance
was necessary. Returning December 2i from a cruise of the
Carnegie^ I found that Dr. Hecker had made reply to some points
in my paper. His remarks were originally published m the
journal of which he is chief editor (Gerland's Beitrage zur
Geophysik, Bd. xi. Heft 1, June, 1911, p. 200); in Novem-
ber a modified translation appeared in this Journal. Refer-
ences throughout this article will be to this translation.

I must begin by correcting some of Hecker's statements.
He infers that we have introduced for gravity work on the
Carnegie an inferior method of reading the boiling-point ther-
mometer, viz., with a hand lens instead of a telescope, as he had
done. I had stated explicitly (J,. <?., p. 4) that the boiling-point
observations on the Carnegie were not made for the purpose of
gravity determinations ; " the prime purpose being to obtain
data for controlling the corrections of our aneroids, the instru-
mental equipment was in accordance with this aim." This was
on our first cruise ; on the present cruise we have replaced the
hand lens by a telescope, but we are not yet willing to regard
our individual results as gravity determinations.



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246 L. A. Bauer — Ocean Gravity Observations.

The experts consulted agreed that it would not be safe to
rely exclusively upon barometers, as did Hecker, damped to
such a degree that, as he confesses, observations made v?ith them
at sea, under supposedly ideal conditions, were not of the
desired accuracy. Thits he says {I. <?., p. 390), " this, of course,
cannot be otherwise, for, as is well known, highly damped
barometers, when perfectly at rest, do not have very accurate
readings." As the final outcome of all conferences and expe-
riences, the conclusion has been reached that it will not be
worth while to take up gravity work seriously on the Carnegie
unless substantial improvements can be made upon the boiling-
point -barometer method; we are continuing, however, with
our present equipment, the necessary observations for the con-
trol of our aneroids. I shall have to postpone for some future
occasion the report upon this feature of our work.*

In spite of all care bestowed, the possibilities of appreciable
errors are so numerous as to raise tne question whether grav-
ity data obtained by Hecker's method would yield individual
results of requisite accuracy. These errors in themselves
appear trivial until converted into gravitational quantities.
Thus, for example, an error in the boiling-point temperature
of but 0°-001 C. corresponds to about 0-035^" or 1/28000 part
of g^ the order of accuracy required,.! am informed, to meet
modem requirements. It may be that Hecker considers that
he has reached this accuracy. As the result of my exami-
nation I was led to the conclusion {l,c.^ p. 161) that "it will
not be surprising if it be found that many of thie most recently
published results are in error by an amount approximating to
0-1^™, or about 1/10000 part of g.''

Hecker questions my statement regarding his thermometer
corrections. The facts as derived from his three publications
are as follows (pp. 6-7, 1903 ; pp. 81-83, 1906, and pp. 39-40,
1910). The total corrections of thermometers employed in the
Atlantic cruise of 1901, the Indian and Pacific Ocean cruise of
1904-05, and in the Black Sea work of 1909, were determined
but once, viz., before starting out in 1901. If I understand
him rightly, only the corrections dependent upon inequal-
ities of bore of tube (the calibration corrections) were deter-
mined a second time, namely, at the end of the work in 1904-5.
All other corrections, however, e, y., those dependent upon the
zero, the fundamental interval, reduction to standard scale, etc ,

* Hecker is correct with rej^ard to the impossibility of reading saccessive
high and low phases of the barometers used on the Carnegie ; 1 had misin-
terpreted the observer's notes. However, since then we have made some
preliminary experiments in which snecessive high and low readings were
obtained by using a hand magnifier and estimating the readings, as closely
as possible, with the eye and attempting to secure the requisite accuracy by
multiplying the observations under varied conditions on the principle suc-
cessfxilly xin^ii in our magnetic work.



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Z. A, Bav^er — Ocean Gravity Ohservations. 247

were ascertained only once. Thus the comparison of Hecker's
thermometers with the standard of the Physikalische Reichs-
anstalt, or with any other standard, were never again repeated
as far as known. Hecker does not give the actual observed
corrections, but, instead, a table computed therefrom, which,
except for very slight corrections due to the second determi-
nation of the calibration corrections, is used for the three
cruises. Hecker assumed that the variations in the mentioned
corrections, with age and use of thermometers, would either be
negligible or "give only a constant difference," and hence
enter into the miscellaneous constant term of his observation
equation. How justifiable these assumptions are I leave others
to judge. Due to the severe conditions imposed by his stren-
uous program, Hecker had serious trouble at times with his
thermometers — sufficient, indeed, to require rejection of some
series. The caliber corrections, I am informed by thermometry
experts, are the ones least liable to appreciable changes, and
from their re-determination no certain conclusions can be
drawn as to the behavior of the other and more important
corrections.

Since Hecker criticises our proposal as to the necessity of
frequent controls of the zero point, it will be of interest to
quote from such an eminent authority on precise thermometry
as Professor Callendar : " The effect [of zero changes] cannot
be calculated or predicted in any series of observations because
it depends in so complicated a manner on the past history and
on the time. It is a most serious difficulty in accurate mercu-
rial thermometry, especially at high temperatures. The most
satisfactory method of correction appears to be to observe the
zero immediately after each reading and to reckon the temper-
ature from the variable zero thus observed." The various
experts consulted in this country are in entire accord with Pro-
fessor Callendar. Now this is what Hecker says {I. c, p. 392) :
"The reason why I made no freezing-point observations is that
they would have introduced new errors into the observations ;
for freezing-point observations are also subject to errors."
Experienced physicists would say that the neglect of the zero
control introduced greater uncertainty than that of a zero
determination. He^er depended too much upon the possibil-
ity of eliminating all outstanding evils by general least square
adjustments ; this same remark applies to other matters referred
to in his comments, e. y., barometer corrections.

A word with regard to Hecker's least square treatment of his
observational quantities. While I have pointed out wherein
his observational work was in some respects not wholly satis-
factory, I am inclined to think that the error due to reduction
will be found to be greater than the purely observational one.



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248 L, A, Bauer — Ocean Oramity Ohseroations,

I have shown (Z. <?., p. 10) that his unknown quantities " are
not strictly instrumental or ship constants, but depend upon
the area (extent and geographic position) from which thej are
derived." Hecker does not appreciate that they can hence
only be used within the area embraced bv the stations entering
into his adjustments and not outside, for extrapolation pur-
poses. For example, in his 1910 revision Hecker assumes that
the unknowns derived from selected stations between the
Tongas and San Francisco likewise hold for the disturbed
region, Sydney to the Tongas. The 1910 computation increases
the gravity anomalies between Sydney and Tonga at times by
O'l^" and more over those of 1908 ; the largest gravity anomaly
of all his cruises, +0'393^, is now placed in this region, viz.,
off the north extremitv of New Zealand. The 1908 compu-
tation, on the other hand, gave as the largest anomaly, +0'319*™,
off Honolulu. The Sydney-Tonga region is that for which
Hecker appeals to Kohlschiitter's paper in confirmation of his
work. Kohlschiitter's own observations were not made on the
ocean but on land, in German East Africa. His general con-
clusion would doubtless hold as well for Hecker's 1908 results
as for those of 1910.

Omitting the rejected port observations, it is found for the
Atlantic work that 44 out of 47 available results were utilized,
whereas for the Indian and Pacific Ocean cruise, out of 136
collected results 65 enter into the least square adjustments for
the derivation of the required unknowns. Those who must
utilize Hecker's anomalies should bear in mind the extent to
which they are already subject to the law of accidental distri-
bution assumed in the adjustments. It may also be of interest
to record here, that for 85 per cent of the total work the appli-
cation of correction due to course and speed of vessel and the
rejection of the port results has increased the sums of the
gravity anomalies squared, the increase being most pronounced
where extrapolated coeflicients have been used.

Hecker has overlooked the salient feature of our proposed
plan, viz., the prime importance of so arranging observational
work as to admit of but one logical method of reduction, and
the necessity of restricting the unknowns to a few physically
determinable ones. I hope that I shall not be regarded as un-
appreciative of his labors. In fact, only one who is himself
engaged in ocean observational work can adequately realize
the countless difiiculties which had to be overcome. My chief
aim has been to assist in setting before those who use his
results their precise limitations.

Washington, D. C, January 22, 1912.



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F. S, Lahee — Metamorphum and Gedogical Structure. 249



Abt. XXIV. — Relations of the Degree of Metamorphism to
Geological Structure ana to Ada Igneous Intrusion in the
Narragansett Basin^ Rhode Island;* by F. H. Lahee.

PabtI.

CONTXNTS.

AcknowledgmentB.
IntTodnction.
Stractaral Geology.

Theoretical considerations.
The borders of the Basin.
The Basin strata.
Major folding
Strikes.
Dips.
Pitch.

Axial planes.

Continuity of the major folds.
Relative number of folds across the Basin.
Minor folding.
Areal distribution of variatioixs in the major and minor

folding.
Conclusions.

Acknowledgments.

For the use of laboratory equipment and for valuable advice
in the preparation of the original thesis, the writer wishes to
express his gratitude to Professor J. E. Wolff and Professor
J. B. Woodworth, under whose direction the work was carried
on ; and for numerous suggestions and favors, to Professors
W. M. Davis, A. Sauveur, and C. Palache; to Dr. Ernest
Howe; and to Messrs. R. W. Sayles, J. A. S. Monks, Wm.
Bums, and W. P. Haynes.

Introduction.

The Narragansett Basin is a body of Carboniferous strata,
fifty miles long, from fourteen to twenty-five miles wide, and
with a total stratigraphic thickness of somewhat more than two
miles.f From the southern coast of eastern Rhode Island it
trends northward as far as a line between Fall River and Provi-
dence, including the major part of Narragansett Bay within its
boundaries, and thence, bending more to the east, extends in a
northeasterly direction to near Hanover, Massachusetts.

Topographically the Basin is represented by a shallow
depression with an uneven surface, between bordering

* The present paper is an abstract of a thesis accepted for the degree of
Doctor of Philosophy in Geology, at Harvard University, in June, 1911.

f Shaler, N. S., Foerste, A. F., and Woodworth, J. B., Geology of the
Narragansett Basin. U. S. G. S., Monog. XXXIII. 1899. See pp. 208-210,
886, 338, 845, 358, and 373, and Plate xzx.

Am. JotTR. Sci.— Fourth Series, Vol. XXXIII, No. 195.— March, 1912.
17



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250 F. H. Lahee — Metdmorphimn and Geological Structure.

Fig. 1.



> ' a » « I t I J .




C*ri6«Bi/tr#itf.
• f a » f



FiQ. 1. Outline map of the sonthem half of the Narragansett Basin.
Many of the dips and strikes are somewhat generalized. References to
numbered localities will be foand in the text.



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F. H. LaJiee — Metamorphism arid Geological Structure. 251

ridgea of harder rocks. A few inliers of the harder rocks
occur surrounded by the Carboniferous (see fig. 1). On all
sides, except in the broken southern rim, the same statements
hold true, namely, that the predominating border rocks are
granites or granite gneisses and that these granites are intrusive
mto sedimentary formations, now much metamorphosed. The
granites (Sterling granite series) in South Kingstown are prob-
ably post-(yarboniferous and the schists enclosed in tnem,
Carboniferous ;* but elsewhere the plutonics are pre-Carbon-
iferous, as is proved by the presence of their disintegrated
debris in the Carboniferous.

The strata of the fiasin are shales, sandstones, arkoses, and
conglomerates, which have been folded, metamorphosed, and
injected by an acid series of dikes and veins, offshoots from the
post-Carboniferous granites of Kingstown. The anticlines are
relatively long and narrow^, after the Appalachian pattern, and
crumpling of minor dimensions often occurs superposed upon
the major folds. Although there are innumerable exceptions,
the metamorphism, regarded from a broad standpoint, is dis-
tinctly greater in the southern part of the field than in the
northern. The acid intrusives range in composition from highly
f eldspathic pegmatites to veins of pure massive quartz. More-
over, they are much larger, and there are many more of them,
in South Kingstown than farther northward and eastward.

Throughout the Basin, then, the texture and composition of
the sedimentary rocks, the complexity of the folding, the
degree of metamorphism, and the composition and abundance
of the acid dikes, are variable factors. It has been our aim to
investigate the kinds and degrees of metamorphism and to
correlate them with the other variable factors just mentioned,
with stratigraphic depth, and with geographic position in the
Basin. The greater portion of the work nas been carried on
in the southern half of the field, where the exposures are more
satisfactory.

The first part of this paper will treat of the Structural Geol-
ogy of the Carboniferous rocks; the second, of the Petrology
and Metamorphism of the Carboniferous rocks; and the third,
of the post-Carboniferous intrusives. There is no need of
describing the pre-Carboniferous rocks in detail. Their impor-
tance for us rests (1) upon their having constituted the floor
upon which the Basin sediments were laid down, and (2) upon
their relations to the forces which deformed these sediments ;
and these matters will be taken up under the other heads.

The remark is perhaps unnecessary that no attempt could be
made to obtain exact quantitative results, because the relations

^Longhlin, G. F., IntruBive Granites and Associated Metamorphid Sedi-
ments in Southwestern Rhode Island, this Joamal, zziz, p. 447, 1910.



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252 F, H. Lahee — Metamorphiam and Geological Str^ccture.

between such factors as mineral composition and intensity of
deformation, or degree of deformation and stratigraphic depth,
and the like, obviously cannot be measured with precision.
Yet a broad, general quantitative dependence can be deter-
mined.



Steuctueal Geology op the Carboniferous Formation.

Theoretical Considerations. — Before describing the struc-
tural geology of the basin, let us see what variations in inten-
sity of folding may be expected, on theoretical grounds, in a
region of deformed strata.

A fold, in the geological sense, is the expression of variable

Fig. 2.



Fig. 2. Diagram of an elliptical quaquaveraal anticline, a-e-6-/, axial
plane.

a-6, direction of operation of minimum component of force,
c-d, direction of operation of maximum component of force.

corapressural forces which have operated upon variable resist-
ances. In most cases the pressure has been applied along
approximately horizontal lines. However numerous the forces
may have been, they may be regarded avS having been equiva-
lent to two components, — one of maximum value, which acted
parallel to the greatest compression, and one of minimum
value, which acted parallel to the least compression, at right
angles to the first.

The simplest illustration is the ouaquaversal anticline (see
fig. 2). Here Or^ is the direction of the minimum component,
and c-d is that of the maximum component. Pitch, measured
along the slopes e-a and e-h^ is really dip in the axial plane,
a-e-b-f. Whenever there is a pitch— and it may be stated as a



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F. n. Lahee — Metcmiorphism and Geological Structure. 253

rule that pitch is practically universal in regions of folding* —
the deforming forces were variable in direction and they
may be considered as resolvable into two components as above
explained. The ratio between these components will then be
inaicated by the ratio between the degrees of compression
perpendicular to, and parallel to, the axis of the given fold.

According to Van Hise,t folding may be parallel or simi-
lar, ■ In parallel folding the contacts between adjacent beds



Fig. 3.



Fig. 4.





Fio. 8. Parallel folding. A-B, fnadamental onrye.
Fig. 4. Similar folding.

are parallel to one another and the thickness of any bed is
essentially uniform throughout (see fig. 3). On the other
hand, in similar folding the contacts between adjacent beds are
identical in size and shape and the thickness of every stratum
is considerably greater in the axial regions than on the limbs X
(see fig. 4). Again, in parallel folding that curve in which
all anticlines and synclines are of et][ual size and shape may be
termeA fundamental to the structure (A-B, fig. 3). Above
the fundamental curve synclines narrow and become ^pinched'
or *carinate', and below it anticlines undergo the same altera-
tion in form. Obviously no such discrimination is possible in
similar folding. It may be shown that, while deformation is
in process, differential movement (shearing) near the funda-
mental curve is at a maximum in the limbs and at a minimum
in the crests and troughs; but that, away from this curve,
both upward and downward, the locus of maximum differen-
tial movement migrates from the limbs to the axial regions,

*See, for example, the following: Reade, T. li., The Origin of Mountain
Banges, xxx, p. 178. London, 1888. Beade, T. M., The Evolntion of Earth
Structure with a Theory of Geomorphic Changes, p. 195. London, 1908.
Shaler, N. S., etc., op. cit, p. 32. Van Hise, C. R., Deformation of Rocks,
Jour. Geol., iv, pp. 812, 844, 848-849. 1898.

t Van Hise, C. R., Principles of North American pre- Cambrian Geology.
U. S. G. S., Ann. Rept. XVI, Pt. I, 1894-1895, pp. 598, 599. Also, by the
same author. Deformation of Rocks, Jour. GeoF., iv, pp. 210, 211, 1896.

JVan Hise, C. R., Principles of North American pre-Cambrian Geology,
pp. 598-801. Also, Heim, A., Untersuchungen fiber den Mechanismus der
Gebirgsbildung, Basel, 1878, p. 48.



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254 t\ H, Lahee — Metamorjphiam and Geological Struciure.

reaching these where the folds are most acutely pinched. In
similar folding maximum shearing is always in the limbs.*

If strata, deformed after the parallel pattern, have a pitch,
carinate anticlines become flat and flat synclines become cari-
uate, when traced in the direction of pitch ; but in similar
folding, since dips are always steeper on the limbs than on the
axes, no amount of pitch can alter these relations. Parallel
folding, representing less readjustment of beds than similar
folding, is more common and is generally on a larger scale
than the latter. It must be remembered that these state-
ments apply to mathematical ideals only, and that, under nat-
ural conditions, there is considerable variation. The two
types are not always sbarply distinguished ; yet there is suffi-
cient approximation to the ideal to make the classification
valuable.

Given a force in operation, a more rigid body will oppose
deformation by this Jcorce more successfully than a less rigid
body. If adjacent rocks of different degrees of rigidity are
under lateral compression, whether the forces be regarded as
acting parallel or perpendicular to the contact surfaces
between the rock masses, there is a tendency for transmission
of these forces by the stronger body.f The first condition —
of force parallel to contact, i.e., about parallel to the beds — is
that for the development of competent structure ;:|: the second
condition — of force about perpendicular to contact — is illus-
trated by the relations between hard crystalline border-rocks
and less resistant basin sediments, after deformation of the
original land surface has progressed far enough. In nature
the differences of rigidity are practically never so great that
one rock merely transmits the force while the other accom-

t)li8hes all the accommodation. Both usually suffer, but one
ess than the other.

The more rigid a rock mass under compressive strain, the
farther from the point of application of the force will the
effects of that force appear. For this reason, unless a stratum
has competency sufficient to enable it to span the breadth of
the deformed belt, the folds are apt to be closer and more
numerous near the point of application and to die out away
from it;§ and the less the rigidity of such a stratum, the
more rapidly will the folds subside.

* Van Rise, C. R., * 'Principles", etc., p. 598.

fHarker, A., On Slaty Oleayage . . . , Rept. Brit. Abboc. Adv. Sci.,
1886, p. 848. Heim, A.: Op. cit., p. 40.

Van Hise, 0. R., Deformation of Rocks, Jonr. Geol., iv, pp. 204, 472,
1896.

t WiUis, B., The Mechanics of Appalachian Stractnre, U. S. G. S., Ann.
Rept. Xin, Pt. II, 1891-1892, p. 247.

§Shaler, N. S., etc., op. cit., p. 18.



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F. H. Lahee — MetamarpMsm and Geological Structure. 255

According to the foregoing review of theoretical facts,
variations in the intensity of deformation may be due (1) to
the type of folding ; (2) to the position of the outcrop in the
fold ; (3) to the degree of rigidity of the rock : and, (4) to the
distance of the outcrop from the point of application of the
force. Provided the proper conditions prevail, then, we
should expect to find such variations in the structure of the
Narragan^tt Basin.

In the description which is to follow, we shall be able
neither to mention strikes and dips of individual outcrops*
nor to debate the pros and cons of questionable intepretations
of the folding.f The method of procedure will be indicated
and then the facts will be presented in summary form.

The Boia>EBs of the Basin. — As may be seen on the map,
the borders of the Basin have many irregularities of trend.



Online LibraryJohn Elihu HallThe American journal of science → online text (page 25 of 61)