THE GEOLOGICAL HISTORY OF NEW YORK STATE 9 
such as limestone which is formed by the accumulation of calcareous 
shells; flint and chert, which are accumulations of silicious shells; 
coal, which is formed by the accumulation of partly decayed 
vegetable matter. Or, finally, they may be formed by chemical 
precipitation, as beds of salt, gypsum, bog iron ore, etc. 
Metamorphic rocks comprise both sedimentary and igneous 
masses which have been greatly changed from their original con- 
dition. Thus, under conditions of great pressure and heat, with 
superheated moisture, sedimentary rocks may lbecome crystalline, 
as when shale is changed to schist, sandstone to quartzite, or lime- 
stone to marble; or an igneous rock may take on a banded structure, 
due to a rearrangement of its component minerals, and thus become 
a gneiss. 
To the modern student of earth science, the old notion of a “ terra 
firma’ is outworn. That idea of a solid, immovable earth could 
never have emanated from the inhabitants of an earthquake country. 
In the recent San Francisco earthquake, along a line of several 
hundred miles, one portion of the Coast Range mountains slipped 
from two to twenty feet past the other. In Alaska, in 1899, a por- 
tion 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 two to twenty feet. 
These are merely striking instances of many of the sudden earth 
movements of recent years. Hundreds of earthquakes occur yearly 
in the islands of Japan alone, and it is probably true that the 
earth is shaking all the time. _ 
There are still other movements which are aoe place more 
slowly and quietly, but which are more significant for our interpre- 
tation of the profound geographic changes which have occurred 
during the millions of years of known earth history. Thus the 
coast of Norway is rising while that of northern France is sinking. 
Distinct beaches at different elevations far above the ocean level 
on the western slope of the southern Andes testify to important 
changes of level in comparatively recent time. 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 still more recently been partially re-elevated 
is indicated by the presence of very young stratified beds of clay 
and sand which are now raised from seventy to three hundred feet 
above the river, the elevation increasing northward toward Albany. 
Actual surveys show that, in the Great Lakes region, a differential 
