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358 F, 77. Lahee — Meiamorphism and Geological Structure.

small ilmenite crystals. After treating a crushed knot with
dilute hydrochloric acid, the residue was found to contain
minute, nicely shaped crystals and reticulated aggregates of
rutile about ilmenite plates.

On the whole the facts indicate that these knots represent
the positions of once existing crystals of some mineral which
decomposed into a pseudomorphic form and was then modified
by addition of material, by recrystallization, and by mechani-
cal rearrangement.

The metacrysts, in pelites of Stage C, named in the order of
decreasing frequency, are, ilmenite, biotite, garnet, and ottre-
lite. They are not all found in all specimens. Generally one
or two are conspicuous and the others are scarce or absent.
These four minerals were formed contemporaneous with, or
later than, the schistosity. Only one among them, the ilmen-
ite, acquires parallel orientation of its crystals in Stage C (see
tig. 2). Sometimes biotite has a linear parallelism such that
the length of its plates are parallel, but the widths lie in all
positions perpendicular to this common direction. Although
isometric and therefore equidimensional, garnet is distinctly a
mineral which develops under dynamic and not under static
conditions. Ottrelite, which is very rare, is wholly unrelated
in size, shape, and orientation, to the schistosity, and is conse-
quently of late anamorphic derivation under static stress.
The infrequent lack of dimensional parallelism in the ilmenite
and the more common absence of the same in the biotite are
due to the fact that these minerals, too, under such circum-
stances, originated under static pressure.

The most highly metamorphosed pelites (Stage D) are char-
acterized by as large a percentage of white mica (muscovite or
sericite) as the composition of the rock will permit ; by a very
high sheen on the fracture surfaces ; and by an excellent cleav-
age. As seen in thin sections, the quartz grains are commonly
elongate (fig. 12) and may be grouped in ribbon-like aggre-
gates. Their shape, their clearness, tneir freedom from strain-
shadows, and their relations to the adjacent minerals, point to
the conclusion that they are secondary or recrystallized quartz.
Both as single grains and as aggregates, they are oriented with
their longest dimensions parallel. Together with the similarly
disposed mica, they are the chief cause for the very good
cleavage.

Among the metacrysts, ilmenite occurs in small plates par-
allel to the schistosity (fig. 3) ; biotite here acquires elongate
habit in single plates and as aggregates, which lie with their
lengths parallel to the cleavage (tig. 17) ; and garnet and ottre-
lite possess the same characters which thev had in Stage C (figs.
18 and 20).



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F, H. Lahee — Metamorphism and Geological Structure. 359
Fig. 4. Fig. 5. Fig. 6.






Fig.



Fig. 8.



Fig. 9.





Fig. 10.




Fig. 11.



c:^/^



Fig. 12.



Figs. 4-12. Changes in quartz, which accompany increasing meta-
morphism. In fig. 4 the grains are still nncmshed and unstrained, and dis-
play their clastic outlines. Figs. 5 and 6 show mottled extinction passing
into granulatioD. In fig. 7 conspicuous granulation is terminal and small
muscovite laths have grown a little way into the ends of the quartz grain.
Fig. 8 represents zonal granulation, due to shearing. Fig. 9 shows both
terminal and zonal granulation and undulose extinction. The curving lines
indicate the direction of schistosity. With complete granulation, flattening,
and some recrystallization, the original grains become flattened aggregates,
as drawn in cross-section in fig. 10. Figs. 11 and 12 are of grains of quartz
which owe their elongate shape entirely to recrystallization.



Fig. 13.



Fig. 14.




Fig. 18. ' Knot ' showing elongation parallel to schistosity and distribu-
tion of sericite, thickly plastered against the sides of the *■ knot,' but less
abundant at its ends.

Fig. 14. * Knot * (partly drawn), showing coarser center and ring-like
arrangement of calcite patches containing limonite (black patches).



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360 F, II. Lahee — Meiamoi'phism and Geological Structure.

Psammites. — The psammites are the commonest rocks in
the Basin formation. They occur at all horizons. As con-
trasted with the pelites, they are coarser, contain less carbon-
aceous matter, ana number feldspar and muscovite among their



Fig. 15.



Fig. 16.



Fig. 17.




^yxy:



Figs. 15-17. Changes in biotite, which accompany increasing metamorph-
ism. Fig. 15 : low metamorphism ; the mica bears no relation to schis-
tosity, which is indicated in the dimensional parallelism of included quartz
grains. Fig. 16 : shreds along the edges of the mica bend into parallelism
with the schistosity. Fig. 17 : aggregates are elongate paraUel to the schis-
tosity (arrow) ; the metamorphism is high.



Fig. 18.



Fig. 19.



Fig. 20.




Fig. 18. Garnet metacryst: The schistosity curves round it. Secondary
chlorite is developed at the poles of compression (right and left). The black
patch is ilmenite.

Fig. 19. Hornblende metacryst, including quartz grains and an ilmenite
crystal.

Fig. 20. Ottrelite metacryst, showing hour-glass structure (due to peculiar
distribution of included quartz and carbonaceous matter of the groundmass).
The black crystals are ilmenite.

original constituents. Those specimens which are least meta-
morphosed (Stage A) already exhibit alteration of orthoclase
to sericite, but this mica has no definite alignment. Both quartz
and feldspar grains still possess their clastic forms (fig. 4).
Flattening of fossil casts and bending of the primary musco-
vite flakes are the only signs of distortion, and these are caused
by superincumbent weight and not by shearing.



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F. H, Lahee — MetaToorphimi and Geological Structure, 861

In many respects the succeeding stages of metamorphiscn in
the psammites are like those in the pelites. Thus, the tokens
of advancing metamorphism, as observed in Stages B to D, are
(1) an increase in the quantity of sericite (here formed largely
from clastic feldspar grains) ; (2) the acquisition of dimensional
parallelism by this sericite; (3) the conseq^uent origin of a
secondary rock cleavage ; (4) the transformation of fragmental
quartz grains, by a process of granulation (see figs. 6 to 9) suc-
ceeding a state of mottled extinction (fig. 5), into lenticular
aggregates (fig. 10) of small, somewhat elongate, parallel gran-
ules, and (5\ the resulting improvement of the rock cleavage ;
(6) the furtner lengthening of the quartz grains by recrystalli-
zation (figs. 11 and 12); (7) the growth of such new minerals
as ilmenite, biotite, garnet, hornblende (fig. 19), and zoisite ;
and (8) the acquisition of dimensional parallelism by ilmenite
and later by biotite, first as aggregates (fig. 17) and then as
individual plates.

These changes take place in the same order and under the
same circumstances as those which were described for the
pelites. Yet, on the whole, any given degree of perfection of
the secondary cleavage is attained somewhat earlier in the
pelites than in the psammites. There is no necessity for
explaining the characters of the several stages in detail.

Psephites. — Although stratigraphically the conglomerates
and their derivatives have as wide a range as the psammites,
their total thickness is less. Their largest expression is in the
Dighton or Purgatory conglomerate, the uppermost member of
the Carboniferous formation in this region.

Consisting of both matrix and pebbles, the psephites may be
studied from two standpoints; out since the matrix, in all
stages of metamorphism, resembles the psammites in the same
stages, we may confine ourselves chiefly to describing the
nature of the pebbles. A considerable majority of these are
of quartzite. Granite and vein quartz are also not uncommon,
and locally, at certain horizons, there are abundant, rather
angular fragments of carbonaceous shale, probably intraforma-
tional. The last are relatively soft, and therefore, as might
be expected, they are somewhat compressed even in the least
crushed specimens (Stage A). As usual, this early phase of
metamorphism is marked by scarcity of sericite and by absence
of a secondary rock cleavage.

Pebbles of Stage B differ from those of Stage A in being
thinly coated with sericite, which gives a silky luster to their
surfaces. At the same time there is more sericite in the
matrix.

A further increase in the development of this mica is exhib-

Am. Jour. Sci. —Fourth Skrdes, Vol. XXXIII, No. 196.~April, 1912.
24

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363 F. H. Lahee — Metamorphism and Oeological Structure.

ited by specimens of Stage C. Here the sericite occurs, not
only in the matrix and plastered outside the pebbles, but also,
to a certain extent, within them. Metacrysts of ilmenite,
biotite, and garnet are sometimes present in the matrix. Many
of the pebbles — even the more resistant ones — particularly
when examined with the microscope, show signs of distortion
in their own oval shape and also in the elongate form of their
individual grains. If two pebbles are in contact, one often
indents the other. The lengths of the pebbles and of their
constituents are roughly parallel to a secondary cleavage
(schistosity) which has been produced in the matrix at this stage.

In Stage D the hardest pebbles may be sheared, flattened or
elongated, bent, warped, and fluted. The less resistant ones
have been squeezed into mere sheets and are cleaved parallel to
their flatness. Recrystallization in pebbles and matrix is at a
maximum. In both parts of the rock sericite is plentiful, and
the metacrysts themselves may be nearly as abundant in the
pebbles as m the matrix.

The study of these phases in the psephites brought out the
fact that spindie-sbapea pebbles (linear schistosity) are charac-
teristic of lower metaphorism than are flattened, sheet-like
pebbles (plane dchistosity). Moreover, when the matrix, as a
whole, is more resistant to deformation than the separate peb-
bles, the pebbles first reveal evidences of incipient metamorph-
ism, and, at any given stage in the history of the rock, they
are at a stage of metamorphism somewhat higher than that
of the surrounding matrix ; and when the matrix is less resist-
ant than the pebbles, the reverse of this statement is true.
That is to say, deformation first affects the weaker portions of
the rock.

Summary.* — The Carboniferous sediments, originally nor-
mal fresh-water elastics, which represented the products of
immature weathering, have been altered by dynamic and static
metamorphism. During the process certain new minerals
developed. In different specimens these minerals vary in
species, in quantity, in orientation, and in size of the individual
particles, and it is found that the variations are consistent with,
and may therefore be used to some extent as indices of, the
degree of metamorphism to which the rock has been subjected.
Other factors, such as perfection of cleavage, deformation of
pebbles, etc., serve in the same capacity. According to these
indices the specimens have been classified in groups, each of
which is characterized by a particular stage of metamorphism.

* In the foregoing outline the major portion of the petrographio inyestiga-
tion, with all its details of megascopic and microscopic description, has
been omitted. We have endeavored to present a fair, though concise, idea
of the kind and amount of metamorphism, and of the method adopted in the
work.



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F, H. Lahee — Metamorphism and Oeological Stmicture. 868

The degrees of metamorphism may now be studied with
reference (1) to rock texture ; (2) to geographical distribution ;
(3) to stratigraphic depth ; and, (4) to deformation. Follow-
ing this, the relations of the schistosity to the bedding, a
subject closely allied to (4), just mentioned, will receive con-
sideration.

Relations of the Degree of Mbtamobphism to Rock
Textube.

A comparison of the degrees of metamorphism in rocks of
coarse and fine grain from different parts ot the Basin would
be unprofitable, tor many other factors might enter to disguise
the true relations. Such a comparison must obviously be one
of purely local significance.

Theoretically, shales, bein^ finer than sandstones, should
yield to folding first ; and, smce folding implies more or less
rearrangement of the rock particles (deformation by flowage
or by minute fracture), metamorphism should also commence
first in the shales. This statement would seem to indicate
that, in a series of strata which differ from one another in
texture, the finer beds would be more intensely crumpled,*
and hence more highly metamorphosed, than the coarser beds,
after a given period of time, metamorphism being in process
all the while.

What are the conditions in the Narragansett Basin ? From
each of twenty-two localities visited in the field work, two or
more specimens of different texture were obtained. Follow-
ing is a list of these specimens :





Conglomerates.
Stage of


Sandstones.


Shales.


General


Stage of


Stage of


locaUty
Just north


metamorphiBm


. metamorphism


metamorphism


of C :1


B


B


C


E : 1


A


A





H :4





A


A


G:5


B


B





C:7





D


C


B:9


C


C


C


£:9





D


c


B : 11





C


c


C : 12





C


D


D : 10





D


D


D: 11





D


C


E : 11





CandD


C


E : 12


D





B



* See remarks on competent structure and reference thereto in Part I of
this paper. This Journal, last number, p. 254.



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364 F, H, LaJise — Metamorphism and Geological Structure,





Conglomerates.


Sandstones.


Shales.


General


Stage of


Stage of


Stage of


locality


metamorphism


metamorphism


metamorphism


H :13


CandB


B





F : 10





C


c


H : 10


D





c


J :5





A


c


J :6





B


A


H :9


C


C





B :U





D


c


D :14


D





c


6 : 14


C





B



According to this table, (1) there are thirteen cases of like
metamorphism in specimens of different texture ; (2) there is
one case of conglomerate having higher metamorphism than
sandstone; (3) there are four cases of conglomerate having
higher metamorphism than shale ; (4) there are seven cases of
sandstone having higher metamorphism than shale; and (5)
there are two cases of shale having higher metamorphism than
sandstone. Thus, out of twenty-seven comparisons, there are
only two instances of the finer rock having the greater meta-
morphism. The same lack of concordance between fact and
theory was also frequently observed in the field.

As for the explanation of this condition, while we realize
that the definition of degree of metamorphism, as used in this
paper, is wholly arbitrary and that the opportunity for error
in assigning such degrees is therefore considerable,' we believe
that the general conclusion which may be drawn will still
remain unmodified. Since the Purgatory conglomerate, the
uppermost member of the Carboniferous formation in this
region, shows evidence of intense metamorphism in the zone of
flow, it is clear that all these rocks must have been under a very
thick cover at the time of their deformation. Probably
adjacent beds, measuring but a few feet in thickness, are not
afi^cted with very marked differentiation according to texture,
under the great pressures existing at such depths. Shearing
is as apt to occur in a sandstone as in a shale. Moreover, as
has been stated by Daubr6e* and by Harker,t more heat is
developed by friction in the interstitial movements of coarse
rocks than in those of fine-drained rocks, and such heat no
doubt assists in the metamorpnic processes.

We infer, then, that, in single outcrops, differences of texture
have little or no direct influence upon the distribution of the
degrees of metamorphism in those outcrops.

*Daubr^, A., Synthetical Studies and Experiments on Metamorphism.
Translation by T. Egleston. Smith. Inst., An. Rept., pp. 463-465, 1861.

f Barker, A., On Slaty Cleayage .... Brit. Assoc. Adv. Sci., Rept.,
p. 848, 1885.



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F, H. Lahee — Meta/morphism cmd Geological Structure. 865

Fig. 21.




Fio. 21. Oatline map of the sonthem half of the Narragansett Basin.
Many of the dips and strikes are somewhat generalized. References to
numbered localities will be fonnd in the text.



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366 F, H. Lahee — Metamorphum and Geological Structure.

Gbogbaphical Distribution of thb Dbgbbbs of Mbta-

mobphism.

Some investigators in this region have noticed differences in
the intensity of metamorphism in passing across belts two or
three miles m breadth. Ijale observed that the Carboniferone
strata have been more metamorphosed on Conanicut and Dutch
islands than in southern Aquidneck Island.* Foerste showed
that the rocks of Prudence Island are less altered than those
of Hope Island.f And both Foerste:]: and Collie§ remarked
upon the increase in metamorphism westward across northern
Conanicut Island. In our field studies we have found a
like advance in metamorphism eastward and westward from
tlie middle north-south strip of the western coast belt
north of East Greenwich ; southward, in the western coast
belt, south of Wickford; and eastward, from Aquidneck
Island to the eastern coast belt. These variations are not uni-
form and, in some cases, are not very conspicuous.

Of more importance is a study of the distribution of the
degrees of metamorphism in the Basin as a whole. This we
have done graphically as follows : The position of the eight
hundred or nine hundred specimens examined was plotted on
the map. For each of these, the kind of rock (coal, shale,
sandstone, or conglomerate) was indicated by a symbol, and
the stages of metamorphism (A, B, C, and D^ were shown by
four different colors. The map was then divided into four
equal rectangles, as in fig. 21, and the allotment of the speci-
mens determined. The results are tabulated below.



Conglomerate
NE. rectangle :

Stage A :
Stage B : 2
Stage C :
Stage D :


Sandstone


8
2



Shale

3

4
2



Coal







Toti

3

14

4




Totals :


2


10


9





21


NW. rectangle :
Stage A :
Stage B :
Stage C :
Stage D *:


2

1
4



5
2

6
1


1
1
2

1


1


1




9

4

13

2



Totals : 7 14 5 2 28

♦Dale, T. N., The Geology of the Month of Narragansett Bay. Proc.
Newport Nat. His. Soc, Doc. 3, 1884, p. 6.

fShaler, N. S., Wood worth, J. B.,and Foerste, A. F. : Geology of the
Narragansett Basin. U. S. G. S., Monog. xxziii, 1899, p, 241.

t Ibid., p. 280.

^ Collie, G. L., Geology of Conanicut Island, R. I. Trans. Wise. Acad., z,
1864-1895, pp. 201, 217.



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F. H, Lahee — Metarruyi^phism and Geological St7*ucture, 367



Conglomerate
S£. Rectangle :

Stage A :
Stage B : 2
Stage C : 10
Stage D : 2


Sandstone

2
9
5



Shale

5

6

10

1


Coal



1
1



Tota

7

18
26

3


Totals :


14


16


22


2


54


SW. Rectangle:
Stage A :
Stage B :
Stage C :
Stage D :


1

5
3


1



7

14


1

2

14

13





1



3

2

27

30



Totals



30



62



The relative number of examples of each stage for each
rectangle may be calculated on a percentage basis :

Rectangle Stage A
NE. 14-286^
NW. 32-143
SE. 12-963
SW. 4-918

Grouping Stages
together, and Stages
together :

Rectangle
NE.
NW.
SE.
SW.



Stage B Stage C Stage D Total
66-667j^ 19-047^ 0^ lOO'OO^

14-286 46-428 7-1435^ lOO'OO

33-333 48-148 6-555 lOO'OO

3-279 42-623 49-180 100-00

A and B (rather low metamorphism)

C and D (rather high metamorphism)



Stages A and B
80-953^
46-429
46-296
8-197



Stages C and D
19-047j<
53-571
53-704
91-803



These figures demonstrate plainly that the degree of meta-
morphism in the southern half of the Basin increases west-
ward and southward.*

Relations bbtwbbx thb Degree of Metamorphism and
Stbatigraphic Depth.

This subject may be approached in two ways : we may study
specimens brought up from known depths in borings or in
mines, or we may examine surface outcrops of which the strati-
graphic position is at least fairly certain.

(1) In regard to the first suggestion. Professor Woodworthf
cited the statement of Professor Collier Cobb, that "the

* We have already snggested that the western coals are most highly meta-
morphosed. See p. 856/
t Shaler, N. S., Woodworth, J. B.^and Foerste, A. F. : op. oit., p. 191.



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868 F. H. Lahee — Metamorphism and Geological Structure.

amount of metamorphism varies with the depth, being greater
at the bottom (of bore-holes) than near the surface." Cobb
drew this conclusion from his study, in 1887, of a set of cores
obtained from two bore-holes in Portsmouth, R. I. (near Loc.
34, H : 9, lig. 21). We have made a thorough re-examination
of the same cores with the following results :

Hole 1 Hole 2



Aotnal


Stage of


Actual


Stage of


depth


Bock metam.


depth


Rock metam.








13'


1"


Coarse sandstone A








36'


1'


Medium *< C








92'


S'


<i « j^








102'


l"


i( U J^








114'


2'


Coarse «* A








129'


3"


Fine, dark shale A








149'


3"


Mediam sandstone B


286' ir


Fine, dark shale


C


194'


1'


Banded shale A


109' lO''


(( i( ((


A














233'


2'


Medium sandstone A


244'


U (C t(


B














284'


r


u u B








325'


w


Coarse shale A








334'


A'


Medium sandstone A


336' 10"


Fine sandstone


B














380'


Z"


Coarse sandstone A


484' 4"


Coarse shale


C








567' 4'


(( ((


C









In Hole 1, if it were not for the first mentioned shale, the
metamorphism would show a regular increase downward ; but
this specimen proves that high metamorphism is not necessarily
limited to great depth in the Basin.

In Hole 2 there is practically no alteration from top to bot-
tom. An exception occurs at 36' V^ where the highest meta-
morphism in the boring is recorded.

Although the data furnished by these specimens are scanty,
nevertheless the facts which they set forth are not to be dis-
regarded. Thev indicate (1) that, at least as low as 400 feet,
depth has no obvious effect upon the intensity of the meta-
morphism, and (2) that a high degree of alteration may occur
near the surface and at considerable depths alike.

(2) According to the interpretation oi the structural geology,
strata in the Wickford district are at about the same horizon
as those of the Bonnet ; yet the metamorphism is more intense
in the latter region. The rocks of Dutch Island (Loc. 14,
C-D : 13, fig. 21), Beaver Head (Loc. 20, C-D : 14), and west-



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F, II. Lahee — Metamorpkism and Geological Structure. 369

ern northern Conanicut, are stratigraphically lower, and are
more metamorphosed, than those of northeastern Conanicut.
In northern Aquidneck Island, the rocks of the eastern and
western coasts are below those of the central portion of the
island ; but the eastern outcrops display more alteration than
do the middle and western outcrops. The conglomerate of
High Hill Point (Loc. 45, 1 : 11-12), correlated with the Pur-
gatory conglomerate of southeastern Aquidneck, is more highly
sheared than the latter. Finally, the arkose and associated
rocks, which are presumably low in the series, are scarcelj'
metamorphosed on the eastern coast of Mackerel Cove (Loc.
22, D : 14), and are not severely crushed on Sachuest Neck
(H:14).

These and other comparisons which might be made, compel
us to believe that tbe degree of metamorphism is not inti-
mately dependent upon stratigraphic depth. As in the case of
texture, this may be on account of the great thickness of the
overlying cover during the process of metamorphism, by virtue
of which differential values were reduced.

Relations Betwbbn thb Degbee op Mbtamobphism and the
Intensity op the Folding.

In describing the structural geology, we discussed strikes,
dips, pitch, axial planes, relative number of folds across the
Basin, and minor folding.* The degree of metamorphism
may now be studied with reference to each of these topics.

Stbikes. — The most regular strikes were said to occur in the



Online LibraryJohn Elihu HallThe American journal of science → online text (page 36 of 61)