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1. The rocks of the Earth's crust are divided into two main
groups.

2. Igneous rocks are composed of particles of pre-existing
rocks.

3. Sedimentary rocks are stratified.

4. Sediments are formed by the action of glaciers.

5. Igneous rocks make up 75 per cent of exposed rocks.

6. Conglomerates are formed as a result of the accumulation
of materials caused by the destructive mechanical action of water.

7. Sandstones are consolidated rocks.

8. Clays are unconsolidated mechanical sediments.

9. Chemical sediments are formed by the destructive chemical
action of water.

10. Peat and coal are the organic sediments which of great
practical value.

11. Clay schist was formed at the beginning of the sedimenta
tion period and clay was formed later.


Unit 4______________________________________________ 93

10. :

1. What main groups of rocks do you know?

2. Do sedimentary rocks consist of particles of pre-existing
rocks?

3. How were igneous rocks formed?

4. Do you know how sedimentary rocks have originated?

5. What is the most important characteristic feature of sedi
ments?

6. Do sedimentary rocks account for 10 per cent of the Earth's
crust?

7. Is gravel a consolidated mechanical sediment? And what about
sand and clay?

What are cementing substances? Can calcium carbonate be
used as a cementing substance?

8. Are there only fine-grained sandstones?

10. What can you say about chemical sediments?

11. Can you give an example of organic sediments? How are they
formed?



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