Preliminary handbook of the department of geology. 23 



disintegrate and fall to the foot of the cliff', often in pieces of consid- 

 erable size. The quartz veins being - hardest and toughest remain intact 

 until the last and often protrude some distance beyond the surface of 

 the schist as shown in the large specimens No. 3903G. 



Once at the foot of the cliff the fragments are alternately thrown upon 

 the beach and dragged back into the sea by each successive wave and its 

 return undertow until gradually reduced to the pebble form. All 

 stages in the process are shown from the angular fragment as it fell 

 from the cliff to the resultant oval pebble. It will be observed that 

 owing to the fissile nature of the schist its pebbles are always in the 

 form of a greatly flattened oval, while those of the massive quartz are 

 more nearly spherical. But of whatever character the material the 

 normal shape of a beach formed bowlder or pebble is oval, and this for 

 the reason that the wave action is a dragging rather than a carrying 

 one; the stone is not lifted bodily and hurled toward the shore to roll 

 back with the receding wave, but is rather shoved and dragged along. 

 Gravity tends to hold the fragments in one position so that the wear is 

 greatest on the side which is down, and this in itself would cause them 

 to assume an oval or flattened form even were they spherical and of 

 homogeneous material at the start. 



At the end of this series is put a sand composed of admixed coarse 

 and fine fragments of shell, schist, and siliceous particles, and which 

 was obtained at low tide further out from the shore. This may be re- 

 gained as illustrative of the material now forming as stratified deposits 

 at this point of the coast. 



Geological action of ice. — Materials illustrating the destructive effects 

 of freezing water are for the time being not separated from those illus- 

 trative of the general process of rock weathering. Here are grouped 

 only objects relating to the phenomena of glaciation as produced by 

 modern glaciers and during the glacial epoch. 



The exhibit begius with a series of photographs taken by Mr. I. C. 

 Russell, of the U. S. Geological Survey, showing the existing glaciers 

 on Mounts Dana and Lyell, in California, and the morainal embank- 

 ments near Mono Lake (70279-70301). There are also shown scratched 

 and scarred pebbles from the Dana glacier (3720G) and a sample of the 

 finely pulverized rock from the foot of the glacier at head of Parker's 

 Creek, near Mono Lake (37234). A similarly formed sediment is also 

 shown from a glacial stream in Greenland (38856). The work of the 

 ice during the glacial period is shown by grooved, polished, and striated 

 stones from several localities. Among the more striking of these atten- 

 tion may be called to the following: A large slab (30 by 48 inches) of 

 grooved and fluted limestone from Kelley's Island, in Lake Erie (38534), 

 and others from St. David's, Ontario (72833 and 72834). These are the 

 slabs figured on pages 194, 195, and 214, Seventh Annual Report United 

 States Geological Survey for 18S5-'8G. There is also a slab (2G by 30 

 inches) from Rochester, New York, showing stria} in two directions, 



