Evolution of the Earth. 87 



thus formed, filling them cand gradually hardening Avithout 

 disturbance, the impressions are j^reserved by the subse- 

 quent hardening of the stone. In this manner have been 

 preserved the foot-prints of enormous birds and reptiles, 

 in the sandstone of the Connecticut valley and elsewhere, 

 specimens of which can be seen in the geological cabinets 

 at Yale College, at Amherst, and at other places. In the 

 lower beds of sandstone we find ripple marks and the im- 

 prints of rain drops, showing that the sand of which the 

 stone was formed was' deposited in shallow water and was 

 occasionally left dry and exposed to the action of rain. 

 These phenomena may also be observed in the sandstone of 

 the Connecticut valley. The same process of depositing 

 sediment goes on, on a much larger scale, at the bottom of 

 the lakes and of the ocean. There is abundant evidence 

 tnat while these layers of comminuted material are being- 

 deposited, the surface of the earth has been subjected to 

 alternate periods of subsidence and elevation. Almost all 

 of the land which now forms our continents and islands 

 was at one time under water ; while much that is now at 

 the bottom of the ocean has, at some period, been dry land. 

 By the accumulation of superimposed deposits and the con- 

 sequent pressure which their weight brings to bear on the 

 lower material, by the elevation of submerged areas, by the 

 gradual infiltration of lime and other saturated material^ 

 and by subsequent chemical changes, the layers of deposit 

 are slowly hardened into stone. The rocks thus formed 

 are naturally not mere solidified heaps of rubbish, but by 

 the action of the water they have been differentiated and 

 arranged in definite, orderly layers, to each of which the 

 geologist gives a name ap})ropriate to its characteristic fea- 

 tures. Last summer, on the to]) of the highest peak of the 

 Green Mountains,* I found the characteristic stone to be a 

 conglomerate or '^ pudding-stone," formed of coarse pebbles 

 and gravel, loosely packed together, showing that the 

 material of which it was formed was originally deposited 

 in shallow water along the shore of some ancient sea. A 

 similar conglomerate constitutes much of the rock around 

 Newport, and along the New England coast at the present 

 time. 



* Killington Peak, near Rutland, Vt., is a few feet higher than Mt. Mansfield, 

 as indicated by the most recent surveys. 



