52 GEOLOGICAL SURVEY 
identical with the forms at present existing on the coast. The fossiliferous portion of the beds does 
not exceed four or five feet in thickness. 
In the northern States beds of gravel, containing rounded and angular boulders, which have 
received the name of “drift,” belong to this formation. The boulders have generally been trans- 
ported from a distance, and have left markings on the rocks over which they passed in their 
journey, which indicate the direction from whence they came. These markings consist of strie 
and grooves on the surface of the rocks. ‘They bear so strong an analogy to similar markings on™ 
the rocks over which glaciers are known to have passed, that they have been referred to the same 
cause. ; 
The glaciers consist of vast fields of frozen snow, covering the sides of mountains and extending 
into the valleys. Whenever the temperature of the air is sufficient to melt the surface of the snow, 
the water thus produced percolates into the glacier, where it is frozen. During this, expansion takes 
place, and the glacier moves onwards, tearing up, like a plough, the surface of the mountains, and 
bearing along with it vast masses of rock, intermingled with clay, earth, and gravel. Rocks are 
frozen into the bottom of glaziers, which score, by the immense weight of the mass, the surface of 
the underlying rocks. When the glaciers reach the point where the temperature is too high for 
them to exist, they melt and leave behind them their immense burden, consisting of the debris of 
the rocks over which their track lay. The melting of the snow and ice often produces floods that 
inundate the neighboring valleys, and have sufficient force to carry forward vast quantities of the 
materials brought down by the glaciers. Such a cause as this has been proposed to account for 
the transported rocks, where glaciers no longer exist. 
The well known transporting power of icebergs has also been proposed. When the vast masses 
of ice that are constantly accumulating during winter in high northern latitudes, break loose from 
the shore, they carry with them masses of rocks and soil, which are distributed over the bottom of 
the sea, as the icebergs melt on their southern passage. This hypothesis, however, does not 
account for the striated surface of the rocks, for as the bottom of the ocean is covered with sand 
and mud, it is difficult to conceive how it could be striated or grooved. Besides, deposits thus 
formed would contain the remains of marine animals, which does not seem to be the case with the 
drift beds. On the other hand, the glacier hypothesis does not satisfactorily account for the vast 
amount of rounded and smooth water-worn pebbles, which abound in these beds, and which 
could only have resulted from the long continued action of water. It is highly probable, then, that 
the force which transported these beds was the result of the combined action of ice and water. 
Alluvium.—This term has been applied to those sedimentary deposits that are going on at present 
on the banks of rivers and along the sea coasts. Such deposits, besides the remains of fluviatile 
and marine animals, sometimes contains works of art and human remains. They are, indeed, the 
only rocks in which the latter have as yet been found. The skeleton found in the West Indies 
some years ago was embedded in a deposit of recent shells, such as is at this moment in progress 
of formation on the coast of Florida. 
The following section will convey a correct idea of the relative position of the rocks composing 
the crust of the earth, so far as it has been examined. 
