10 NEW YORK STATE MUSEUM ' 



are accumulations of siliceous shells ; or coal, which is formed by 

 the accumulation of partly decayed organic matter. Or, finally, they 

 may be formed by chemical precipitation, as beds of salt, gypsum, 

 bog iron ore, etc. 



Metaniorphic rocks include both sedimentary and igneous rocks 

 which have been notably changed from their original condition. 

 Thus, under conditions of moisture, heat and great pressure, sedi- 

 mentary rocks may become crystalline, as when shale is changed 

 to schist, sandstone to quartzite, or limestone to marble; or an 

 igneous rock may take on a streaked or crudely banded structure 

 due to more or less flattening and rearrangement of its component 

 minerals, and thus become a gneiss. 



To the modern student of earth science the old notion of " terra 

 firma " is outworn. That idea of a solid, immovable earth could 

 never haye emanated from the inhabitants of an earthquake coun- 

 try. Earthquakes are caused by sudden movements in the crust 

 of the earth, most commonly resulting from slipping of one part 

 past another along a fracture. Such fractures, knovvoi as " faults," 

 are very common in the southeastern half of the Adirondacks. 

 In the San Francisco earthquake of 1906, along a fracture line of 

 several hundred miles, one portion of the Coast Range mountains 

 suddenly slipped from two to twenty feet past the other. In Alaska 

 in 1899, a portion of the coast was bodily elevated forty-seven feet. 

 In Japan in 1891, for a distance of forty miles along a rift in the 

 earth's crust, there was a sudden movement of from ten to forty 

 feet. These are merely striking instances of many of the sudden 

 earth movements of recent years. It is probably true that the 

 earth is shaking some place all the time. 



Still other movements take place more slowly and quietly, but 

 they are very significant for our interpretation of the profound 

 geographic changes which have occurred during the millions of 

 years of earth history. Such movements are now known to be 

 taking place. Thus the northern portion of Sweden is rising sev- 

 eral feet a century, while the southern portion is sinking. A fine 

 illustration of notable sinking of the land is proved by the drowned 

 character of the lower Hudson valley, and by the fact that the old 

 Hudson channel has been definitely traced as a distinct trench in 

 the ocean bottom for one hundred miles eastward from Sandy 

 Hook. That this same region has been more recently partially 

 reelevated is indicated by the presence of very young stratified clay 

 beds and sands which are now raised from zero near New York 



