Gateway to the Classics: Handbook of Nature Study: Earth and Sky by Anna Botsford Comstock
 
Handbook of Nature Study: Earth and Sky by  Anna Botsford Comstock

Granite

Teacher's Story

In granite, the quartz may be detected by its fracture which is always conchoidal and never flat; that is, it has no cleavage planes. It is usually white or smoky, and is glassy in luster. It cannot be scratched with a knife. The feldspar is usually whitish or flesh-colored and the smooth surface of its cleavage planes shines brilliantly as the light strikes upon it; it can be scratched with a knife but this requires effort. The mica is in pearly scales, sometimes whitish and sometimes black. The scales of these mica particles may be lifted off with a knife, and it may thus be distinguished. If there are black particles in the granite which do not separate, like the mica, into thin layers, they probably consist of hornblende.

Granite is used extensively for building purposes and for monuments. It is a very durable stone but in the northeastern United States where there is much rain and cold weather, the stone decays. Mica is the weakest, hornblende next, and feldspar is next to quartz, the strongest constituent of granite. Water permeates the mica, hornblende, feldspar and sometimes the quartz, and by its expansion in freezing causes the stone to crumble. The reason why polished granite endures better than the rough finished, is that the smooth surface gives less opportunity for the water to lodge and freeze. When the weathered granite is cut up into small particles by the waters of streams, they are sifted and all the parts which are soluble are carried off, leaving a sand composed of quartz and mica, which are insoluble. This sand is washed by streams into lakes, and then is dropped to the bottom; if enough is thus carried and dropped, it forms sandstone rock. All of our sandstones used for building purposes were thus laid down.


[Illustration]

The granite obelisk still standing on the site of the ancient city of On.

Photo by J. H. Comstock.

Cleopatra's Needle, which stood for thousands of years in the dry climate of Egypt, soon commenced to weather and crumble when placed in Central Park, N. Y. This shaft has a most interesting history. It was quarried near Assuan, in the most famous of all granite quarries of ancient Egypt. It was cut as a solid shaft in the quarry and carried down the Nile River for 500 miles—an engineering feat which would be hard to accomplish to-day, with all our modern appliances. It was one of the obelisks that graced the ancient city of On, later called Heliopolis, situated on a plateau near the present city of Cairo; On was the city where Moses was born and reared. There is still standing where it was first placed as a part of a magnificent temple, the temple a part of a magnificent city, one of these obelisks. It now stands alone in the middle of a great fertile plain, which is vividly green with growing crops; a road shaded by tamarisk and lebbakh trees leads to it; nearby is a sakiah, creaking as the blindfolded bullock walks around and around, turning the wheel that lifts the chain of buckets from the well to irrigate the crops; and a hooded crow, whose ancestors were contemporaries of its erection, caws hoarsely as it alights on the beautiful apex of this ancient shaft, which has stood there nearly four thousand years and has seen a great city go down to dust to fertilize a grassy plain.

Lesson CCXVII

Granite

Leading thought—Granite is composed of feldspar, quartz and mica, and often contains hornblende.


Method—Specimens of coarse granite and a pocket knife are needed.


Observations—

1. What minerals do you find in granite? How can you tell what these minerals are? Look at the granite with a lens. How can you tell the quartz from feldspar? Take a knife and scratch the two. Can you tell them apart in that way? How can you tell the mica? How can you tell the hornblende?

2. What buildings have you seen made of granite? What monuments have you seen made from it?

3. Which mineral in granite is especially affected by water? Which remains unharmed the longest?

4. What is weathering? Mention some of the characteristics of weathering. Why does the rough-finished granite weather sooner than that which is polished?

5. Examine some sand with a lens. What mineral do you find present in it in the greatest quantity?

6. Write the story of the Cleopatra's Needle in Central Park, New York City.


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