126 NEW YORK STATE MUSEUM 



Schist, mica schist, hydromica schist, talcose schist, etc. 

 Various members of the mica group play an important part in 

 the schists. 



Slate. This is mainly shale hardened by metamorphism and 

 rendered fissile by pressure. The roofing slates are good ex- 

 amples. 



Crystalline limestone. This is sedimentary limestone made 

 crystalline by heat and pressure. 



All kinds of igneous rocks may become schistose by metamor- 

 phism and then receive names indicating their composition and 

 structure. 



HISTORIC GEOLOGY 



Historic geology treats of the succession of geologic deposits 

 and is based on the study of sedimentary rocks. 



It is estimated that the geologic series consists of about 

 100,000 feet or 20 miles thickness of sedimentary strata. These 

 are beds of sediment chiefly formed by successive invasions of 

 the sea and the transportation and deposition by it of debris de- 

 tached from the rocks of the mainland by rain, frost, rivers and 

 the ocean waves. 



It has been estimated that about 99^ of all rocks are sedi- 

 mentary, and although some of these were formed in fresh 

 water, probably the larger part of the sed : :nentary rocks were 

 deposited in the ocean. It has consequently been said that 

 'the sea is the mother of continents.' On our Atlantic coast, 

 as elsewhere, the ocean is both a destructive and a formative 

 agent. As the soundings show, the loose materials washed from 

 the land are spread out about 100 miles from the shore line 

 in a broad, sloping plain of sand and mud. In such submarine 

 deposits, when uncovered by the ocean's retreat, we find the 

 remains of mollusks, fishes and other marine forms of life. Be- 

 sides, land animals are often drowned and their bodies are carried 

 out to sea and covered with sediment, leaves fall on the water 

 and sink to the bottom. Therefore, in rocks formed in the sea 

 we sometimes find remains of land animals and plants, besides 

 the marine forms which we expect. The unceasing action of 



