368 Frank Carney 



period is changed to consolidated rock without ever being carried 

 to the seas. There was a time when students thought the sedi- 

 mentary rocks all represented material that had been deposited 

 in, or near, the oceans. 



Ocean sediments. On the other hand, ocean waves and cur- 

 rents are contemporaneously engaged in eroding the rock along 

 parts of their borders, and in further refining .the river deposits. 

 Thus the oceans may be constantly sawing into the continents, 

 pushing their cliffs gradually inland, thus contributing materials 

 for sedimentary rocks of the future. The waves undercut the 

 cliffs, pound up and prepare a load which the ocean currents 

 carry away, assort, and deposit. Our seashores then are another 

 belt where sedimentary rocks are in-the-making. Nevertheless 

 a conspicuous part of the sedimentary rocks are associated directl}- 

 with river basins. 



But how may we so distinguish the rocks of the past that we may 

 be sure which of them represent old seashores and which flood 

 plains? It is not an impossible task. Careful study has been 

 made of the prolific life of the ocean border tract, and the some- 

 times equally abundant life of the river flood plains. These 

 faunas in the main are quite distinct: on the one hand marine, on 

 the other, fresh water. When the animal dies its tissue is some- 

 times replaced by minerals, producing a fossil. In the fossil form 

 these organisms ma^^ be studied and classified. Therefore it is 

 usually possible to tell whether a certain horizon of rock repre- 

 sents fresh water or marine sedimentation. 



THE ORIGIN OF ROCKS 



Old ideas as to the origin of rocks have been somewhat mod- 

 ified in recent years, because of new teachings of astronomers 

 and physicists in reference to the origin of the earth itself. There 

 are two more widely accepted hj'-potheses of earth-origin. I will 

 state briefly the parts of each which bear more directly on the 

 genesis of rocks. 



Nebular Hypothesis. The early conception of our globe pic- 

 tured its gradual evolution from a spheroid of gas which slowly 

 condensed, acquiring great heat, and forming at last a molten 

 body about the size of the present earth. As heat was radiated 

 from the surface of this hot spheroid, a cool crust developed; 



