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Ralph S. (Ralph Stockman) Tarr.

A laboratory manual for physical and commercial geography online

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Purpose.



Steam a
cause of
porous
lavas.



XIX. — CLASSIFICATION OF IGNEOUS ROCKS

For Each Student. — A specimen of pumice, cellular lava, obsidian, rhyolite, granite, trachyte,
syenite, gabbro, basalt.

For General Class Use. — Small pieces of rock, unlabeled. .

To become familiar with the general types of igneous rocks, and the characteristics by which
they are distinguished.

When water is boiling, in what form does the steam rise from the bottom of the vessel to

the surface ? What is meant when we say that the

liquid which is boiling "froths over" ?



If this " froth " were hardened just as it rises over the rim of the vessel, what would be the
appearance of the mass formed ?



Classifica-
tion by
texture.



If molten rock were being " frothed over," or blown out of the crater of a volcano by the
steam and gases that rise from it (see Fig. 203, Text-book), what would be the nature of the

rock that is formed on its cooling?

Which of your specimens shows evidence

of having had such a history ? Which one shows

evidence of less violent steam action ?

If the molten rock flowed over the volcano rim very quietly (like molasses candy being
poured from a kettle), and then cooled quickly, which of your specimens do you think it

would most resemble ? If there were no steam

or gas in the lava, which specimen would result?

Why is artificial glass similar in appearance to this specimen?



85



Find the specimen which you think has cooled a little more slowly than the obsidian.



What specimens give evidence of the following condition of cooling : First, the lava cooled
slowly for a time, at a rate which permitted some of the minerals to separate out and form
visible crystals ; then it cooled rapidly and the rest of the mass made a rock of fine texture ?



What minerals have separated out in your specimens of these (porphyritic) rocks ?.



Which rocks formed in the places indicated in Fig. 12 would cool rapidly ?.




Fig. 12. — To illustrate the Origin of Igneous Rocks.



Would this give rise to coarse or fine grained rocks?

In what position would such rocks as granite, syenite, and gabbro be formed ? .



Classifica-
tion by
composi-
tion.



The observations you have made above give a basis for a classification of igneous rocks
by structure, or texture. Pumice, obsidian, and some other lavas are glasses. Khyolite and
trachyte are porphyries. Basalt is a finely crystalline igneous rock. Granite, syenite, and
gabbro are coarsely crystalline igneous rocks.

The glasses cannot be further subdivided (because they are too fine grained) ; but all the
other rocks are also grouped according to their composition.

Refer to your table of the properties of minerals. Which minerals are light in color ?



86







Which are dark ?



In which of these two groups are the minerals which have the most

metallic elements (iron, magnesium, calcium, sodium, and potassium)?

' Which is the darker,

orthoclase or plagioclase feldspar ? Which has the most metals

in its composition? What do you observe as to the relation be-
tween color of mineral and the presence of metallic elements ? 1



Arrange all your specimens

(except the glasses) in a row, putting those lightest in color at the left-hand side. Write

down the order in which you have arranged them.



What sort of a rough grouping have

you now made of the rocks ?



Now make two rows of your specimens, putting the porphyries and basalts in the lower
row, but still keeping the specimens in the same order as to color. Write down the order of

the specimens in each row.



87



Examine your granite specimen (which should be first in the upper row). What minerals

does it contain (look very carefully) ?

Examine the syenite (the next in order). What

prominent mineral of the granite does it lack? Which kind of

feldspar has it ? Examine the gabbro. Which kind of feldspar has

it? What other minerals does the gabbro contain ?



A rock is said to be acid when it is made up mainly of the light-colored minerals, which
have the lesser amounts of the metal elements. The rocks in which the minerals have metallic
elements are said to be basic. Thus quartz is the most acid, and pyrite the most basic of
those rock-forming minerals that you have studied. Which of the rocks in your collection

are acid ?



Which basic ?



Whether a rock is acid or basic gives a basis for a second method of classifying igneous
rocks ; namely, a classification according to composition. In the following table write in the
names of each of your specimens, keeping in mind the two kinds of classification : — (1) com-
position; (2) texture.



88





ACID END




BASIC END


>-






Has quartz and or-
thoclase feldspar


Orthochase feldspar
but no quartz


Has plagioclase feldspar
and other basic minerals


Coarsely
Crystalline








Finely
Crystalline






Basalt


Porphyries








Glasses


.



By means of this table identify the specimen you have collected from the home locality if it belongs
among the Igneous Rocks. Identify the small pieces of rock given you by the teacher, telling (a) the minerals
in each ; (6) whether acid or basic ; (c) its texture ; (d) its name.



t



89



Materials.



Purpose.



Sedimen-
tary and
organic
rocks.



XX. — CLASSIFICATION OF SEDIMENTARY, ORGANIC, AND
METAMORPHIC ROCKS

For Each Student. — Specimens of conglomerate, sandstone, shale, limestone, coal, gneiss,
schist, quartzite, slate, marble.

For General Class Use. — Hydrochloric acid and glass stirring rods. Small pieces of rock,
unnamed.

To become familiar with the more common types of these classes of rocks, and to learn how to
distinguish between them.

Assume a stream flowing into the sea and carrying along in its current small pebbles,
sand grains, and clay particles. It also has carbonate of lime in solution. All this material
the stream has secured along its course by the breaking up of some kind of rock, through the
action of the agents of weathering and erosion.

When such a stream empties into the ocean, will its current continue unchecked in

velocity? ___. Will the current continue at all for any great distance

from the shore? What then will become of the rock material that the

current is carrying?

Which part of the load that the current carries will be deposited first?

Why? ._







Which of your specimens is formed of such

materials ? Which kind of material will be

deposited a little farther out from the shore? '.

What kind of rock does this make ? Which of your specimens might

result from the deposit of the clay particles ?

Which of your specimens might have resulted from the deposit of the lime solution?

How might the sea animals, whose traces

are shown in your limestone, aid in its formation? .



91



Would there be a sharp line between the place of deposit of sandstone and conglomerate ?
Between the sandstone and the shale? Draw a diagram



illustrating deposit of sediment in the sea to show how variation in texture would occur
from the shore outwards.



Limestone is often formed from the remains of lime-secreting animals. Plant remains also
form organic rocks. Coal is such an organic rock formed of plant remains accumulated in

ancient swamps. Are there any traces of plants left in your specimen? How

could you test whether a black rock was coal ? 1 - ,

In what ways do sandstone and limestone differ?



Shale and limestone (use acid test) ?



Shale and sandstone?



92



Sandstone and conglomerate?



Identify the unlabeled specimens.



Metamor- By means of the acid test find which of your metamorphic specimens was changed from

phic rocks.

a limestone. Compare it with limestone and

note the natur*e of the change.



Which of the metamorphic rocks most nearly resembles shale?

How does it differ from shale?



Micaceous minerals are abun-
dantly present in slate, and you may be able to see them in your specimen. Why would the

presence of such minerals cause the slate to split so regularly?







What two specimens have

been so metamorphosed as to resemble the igneous rocks ?



93



Assuming both these to have been originally clay rocks, which has apparently

been changed most? What is the arrange-
ment of like minerals in each?

Why is it easier to split the schist than the gneiss ?



What metamorphic specimen is yet unclassified? From what is

this derived? How has it been changed?



Identify the unlabeled specimens. If the specimen of rock from your home locality was not an igneous
rock, identify it among the sedimentary, organic, or metamorphic rocks.



94



C



#



Materials.



Purpose.

Origin of
soil.



XXI.— THE SOIL

For Each Student. — Specimen of rock with lichen attached. Several rounded stream pebbles.
Residual soil from granite. Residual clay from limestone. Field soil secured from beneath sod.
Small fragment of limestone. Hydrochloric acid. Test tube. Glass plate.

For General Class Use. — Hand specimens of minerals and rocks.

To study the origin and nature of soils and their significance to man.

From your study of minerals, what few make up the larger part of the rock masses of

the earth's crust?

Which of these minerals have cleavage planes?

If you poured water over minerals, some with cleavage planes and some

without, which ones would absorb the water most readily ?

What would happen if the water-soaked minerals were to freeze ?

, How would a

rock composed of cleavable minerals be affected if first soaked by rain, and then exposed to
a freezing temperature?



When the mercury in a thermometer tube is heated, why does it rise ?



Most substances have this property, but the amount of expansion

for the same degree of heat is different for different substances. Thus different minerals
expand different amounts under the same heat. What would be the effect on the rock itself

if the minerals in a granite expanded at different rates?



If you put salt in water, what happens ?



If you put acid on limestone, what happens ? 1

Other acids have a similar effect. Rain water obtains carbon dioxide from the air and from
plant remains. The combination of the water and carbon dioxide forms carbonic acid. When

95



this acid soaks down to the soluble limestone, what will happen ?



Examine the rock specimen with the lichen growing on it. By what means does the

plant cling to the rock ?

If its roots grow after penetrating a tiny crevice in the rock, what
will be the effect of their growth on the rock ?



State clearly the general effects of these weathering agents on all minerals and rocks
exposed to the air.

Have all minerals cleavage planes ? Are all minerals

dissolved by acid? Are all rocks made up of the same amounts, or of the

same kinds of minerals ? What bearing have these differences on the rate at

which different rocks are broken down ?



Why is almost all the earth's surface covered with

"dirt," sand, and clay, while bedrock is seen only occasionally ?



Examine the stream pebbles. Have they sharp edges and corners?

What has happened to the pebbles since they were broken off from the bedrock ?



What becomes of the particles which are ground off?

What kind of deposits do they form in

stream beds ?



96



Residual Examine the residual soil from granite by spreading a small portion on your glass plate.

soils.

What mineral of the granite has remained unaltered? What mineral

composes the greater mass in a granite rock?

What material seems most abundant in this residual soil of granite?

What has happened to the feldspar of the granite?

Put a small fragment of limestone in a test tube half full of hydrochloric acid. What

happens ? What sort of material remains in

the test tube after the bubbling ceases? _.

Examine the residual clay from the limestone.

How does it differ in composition from the residual material of granite?



Why is there

this difference?



The original limestone rock from which the clay has been derived may

be gray, brown, or black. What is the color of the residual clay ?

To what is the color due?

What material of the limestone rock remains behind as residual clay ?

Which rock would

give the greater amount of residual material, granite or limestone?

Why?



97



il- Examine the sample of field soil. How does it differ in appearance from the pure resid-
ual soils?



Put a small portion of the soil in a test tube f full of water ; shake, and then allow to settle.

What kind of material floats? What is the

source of the organic matter? :

What part of the soil remains suspended for a long time?

What material quickly settles to the bottom ?

How does the field soil differ in composition
from the pure residual soils?



What is the effect of the presence of plant roots on the texture of the field soil?

The presence of organic matter, and the
greater porosity of field soils, gives them fertility and enables them to support plant life.
The subsoil, in which plants have never grown, is quite infertile. Why then would it be
unwise to plow up soil below the depth to which plant roots usually penetrate ?



Why are manures placed on poor soils ?



Where do growing plants secure their water ?



Will plants grow well in a very compact soil? (See suggested home experiment.)
What is the effect of plowing the soil ? ,.



W



98



What kinds of food do cattle eat? What

kinds of food does man eat? Traced back

to its original source, what is the real basis of supply of food for man ?

State clearly the

importance of soil to the life of human beings.



Suggested Secure four pans about three inches deep and eight inches long. Into two of these put clayey field soil,

home or loosely, and in each, during the process of filling, plant 20 grains of corn at a depth of one inch. Keep one

general of these pans moderately moist, the other very wet, and let both stand in a warm place.

class work. Into the remaining two pans put the same kind of field soil, but pack it very firmly ; and, during the

process of filling, plant 20 grains of corn in each at a depth of one inch. Keep one of these moderately moist,
the other very wet. After ten days observe how many plants are growing in each pan. Under which treat-



ment did the greatest number of seeds grow ?



The least ?



What was the effect of the different treat-



ments of the soils on the entrance of air between the soil grains ?







What is the effect on plant life of pre-



venting the entrance of air into the soil ?



Give a reason why fields are plowed.



Why a field should be



drained of its surplus water.



99



(



A



i



SCALE.
Fig. 13. — Contour Interval.



OF THE

UNIVERSITY

OF
s£4UF0RNVf



MAKING AND INTERPRETATION OF TOPOGRAPHIC MAPS

XXII. — CONSTRUCTION OF AN AREAL MAP

(With tank and land model, see page viii)

Materials. For Each Student. — Ruler. Pencil (well sharpened).

For General Class Use. — Land model and tank. Yardsticks.

Purpose. To make an areal map of a miniature land form.

Scale. What is the length of the platform on which the land model rests?

What is its width? What is the length of the accompanying sheet

of paper (Fig. 13) ? Its width? If your

platform were five feet long and four feet wide, and your paper ten inches long and eight
inches wide, what would be the greatest amount of space that you could allow for every foot of
the platform, if you were asked to draw an outline of the shape of the platform on your paper ?

. What explanation would you need to put on

such a drawing so that others could understand what the size of the original was ?

What

is meant by the scale of a drawing or map ?



Why would it be more convenient to adopt a

scale one half that of the largest possible scale for drawing the 5x4 foot platform on paper

10 inches by8 inches in size?

What does the phrase 1 inch = 1 mile (printed on many maps)

signify ?



From your measurements of your own platform and paper, decide what will be

the most convenient scale for you to adopt for the map you are to make, and write it down
opposite the word " scale " on the sheet of paper (Fig. 13).
Orientation Turn your drawing paper so that the arrow printed or* the sheet points to the north.

Mark an N above it. Mark S for south, and E and W at the proper ends of the other lines.

Which side of the model will you draw on the north side of your map?



Holding a map so that its directions correspond with the actual

directions is called " orienting " the map. Why should this be done, whenever possible, when

101



using a map of any kind ?



Areal
mapping.



Draw an outline of the platform according to the scale you have adopted. Then beginning
on the side assigned by the teacher (different members of the class begin on different sides),
measure with the yardstick the distance from the edge of the platform to different points
where the water touches the land model (as a-b, Fig. 14). Measure only to the prominent
points, such as the ends of the capes, the heads of the bays, etc. Measure also, each time, the




WATER



MEASUREMENT OUT
- TO LAND FORM



MEASUREMENT ALONG PLATFORM



=*



16



Fig. 14. — Diagram to show Method of making Measurements for Areal Map.

distance from a corner of the platform along the edge of the platform to the point from which
you measure out to the land model. These two measurements are diagrammed in Fig. 14 as
a-b and b-c.

Then, in the same way, locate the position of the highest points of mountains, and mark
these on your map by little triangles ( = A ). Next, locate the course of the main streams (as
indicated by blue yarn on the model), and trace a line on the map to show each of these. Lo-
cate other points as indicated by the teacher.

The work you have done so far is similar to the work that topographers do when making
an areal map of a similar island in nature, except that in actual land mapping you would
measure with instruments from point to point on the island. Also, you would need to locate
more points. Areal mapping means locating the position of prominent points, streams, roads,
etc., in a given area, with reference to each other and to the cardinal directions ; and also de-
termining their latitude and longitude. Could you tell the height of a mountain from such an

areal map? Could you tell whether a mountain had a steep slope on one

side or a gentle slope on the other? Could you determine the form of a

mountain ridge ? What two features shown on the model are not expressed

on the map as you have made it up to this point ?



102







Materials.



Purpose.

Base level,
or datum
plane.



XXIII. — CONSTRUCTION OF A CONTOUR MAP

For Each Student. — Ruler and brown pencil, well sharpened.

For General Class Use. — The model used in the previous exercise [and if plaster model is
used, a number of boards of uniform thickness as directed on page ix of this manual].

To express the relief features of a miniature land form on a map by means of contour lines.

What line on the areal map that you have drawn (Fig. 13) shows the contact of water

and land?

What is its level, with reference to the water surface?

What kind of surface has a body

of standing water? Why are all elevations

on the earth's surface expressed by their height in feet (or meters) above sea level ?



Why is the mean (or average) sea level often referred to as the datum plane, or base level ?



Contour
interval.



The water in the tank may be considered as a miniature sea.

What is the elevation of any point along the outline of the map you have drawn, with

reference to this sea level, or datum plane ?

. Lower the model and its platform a certain

depth into the water, equally on all sides. How much did you lower it ?

Has the water level changed? Where does the water surface come into

contact with the land now ? (Express in general terms.)



103



Draw (with brown pencil, on Fig.

13) the new outline of the contact of the land surface and the water (making measurements
and plotting, as done previously on the areal map you have made). Does this new outline

come outside or inside the original outline ?

If now the model were lifted back to its original position with reference to the water level,
and a path were traced around the model in the position fixed by the second outline, would

this path go uphill, remain always at the same level, or go downhill ?



"What then is the



relation of the second outline to the first outline ?



Such a line, drawn on a map through all

points at the same elevation above sea level, is known as a contour line. Why should the dif-
ference in elevation between the sea level and the contour line you have drawn be known as a

contour interval ?



Contour Does the contour line that you have drawn extend farthest in from the sea-level line at

lines and points where the slope of the model is steep or gentle ?



slope.



Succeeding
contours.



Lower the model again, the same distance as the first time. Locate and plot the new

contour line. What is the contour interval you have adopted ?

Write it down on the line opposite the words "Contour Interval" printed

on your map (Fig. 13). Number your contour lines on the map to show what elevation
above the original sea level they represent. Lower the model equal distances until the
highest points are submerged, and draw the contour lines for each interval. What is the form

of the cdntour line showing the highest elevation of the mountain ?

How do the contour lines bend when they enter a valley?

W 7 hen they come to a headland? ___2

"Where are they farthest apart, on steep or gentle slopes?

What reason can you give for using a brown pencil

in drawing the contour lines? _



Use of a
contour
map.



W 7 hat does a contour map show that the areal map did not ?



Why are such maps



104







called topographic maps?

Why would a topographic map be much more useful than

a simple areal map to a man who wished to make a road between two points ?



How could one find out from a topographic map how many feet a river descended from its
source to its mouth?



105







Materials.
Purpose.



XXIV. — MAKING A CROSS SECTION OF A CONTOUR MAP

For Each Student. — Ruler. Sharp pencil. Sheets of plain paper.

To teach the making of cross sections from contour maps.








Fig. 15. — Contour Map of Miniature Land Form made by a Student in Physical Geography.

Study of a Figure 15 is a contour map of a miniature land form made by a student in physical

simple con- geography,
tour map.

What is the scale of this map?

107



What is the contour interval? How can you tell which slopes on

the land form were steep and which were gentle?

: On which sides of the miniature land form

were the shores steep?

On which sides did it have gentle slopes?

, • What was the

highest elevation on the land form? Why was this figure printed

in on the map? T

Which contour lines are heavier than others?

What aid does this heavier printing of some contour lines give when

reading the map?

By what other means are

these heavier contours distinguished from the rest?

On the northern side of the map, the two-inch contour line bends far inland. If you crossed
the miniature land form in an east and west direction, would you find the land inside the

bend higher or lower than two inches?

What would be the case if the contour bent outward toward the sea?

Find and note an illustration of a contour line bending outward.

Meaning of Note the line A-B on the map, Fig. 15. What are the elevations of the two highest points

cross

section. it crosses ? What is the elevation of the lowest

point it crosses between these high points ? Make a drawing (on Fig.

16) to show how your path would go uphill and downhill if you were to cross the land form
along the line A-B, starting at A.



-x



A B

SEA-LEVEL Fig. 16. SEA-LEVEL

The drawing you have made is a crude cross section of the land form along the line A-B.
If a cross section is accurately made from a contour map, it shows clearly, in diagram form,

108



Plotting
| intersec-
tions of
contours.



the elevations, depressions, and level places of the region along the line on which the cross
section is made.

To make an accurate cross section along the line A-B proceed as follows : Fold a sheet of
your plain paper through the middle so that you have a smooth, folded edge at least six inches
long. Next lay this paper over the map so that it covers the southwest corner of the map, and


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Online LibraryRalph S. (Ralph Stockman) TarrA laboratory manual for physical and commercial geography → online text (page 5 of 16)