THE FRIENDLY ROCKS 



sandstones in deeper water, and the slates and argil- 

 laceous rocks in deeper still. The limestone rocks, 

 which are of animal origin, also imply deej), calm 

 seas during periods that embrace hundreds and thou- 

 sands of centuries. It is, then, the long ages of peace 

 and tranquillity in the processes of the earth-build- 

 ing forces that have contributed to the homogeneity 

 of the different systems of secondary rocks. What 

 peace must have brooded over that great inland sea 

 when those vast beds of Indiana limestone and sand- 

 stone were being laid down ! A depth of thousands of 

 feet of each without a flaw. Vast stretches of Cam- 

 brian and Silurian and Devonian time were appar- 

 ently as free from violent movements and warrings 

 of the elements as in our own day. 



Occasionally in a system of rocks one may see a 

 change of color over a considerable area, as from 

 gray or brown to red, with small fragments of older 

 and redder rocks embedded in them. I fancy such 

 streaks were caused by a sudden flood or freshet that 

 carried new material worn from a distant land-sur- 

 face into the sea or into the impounded waters. 



It would seem to require as distinctly an evolu- 

 tionary process to derive our sedimentary rocks 

 from the original igneous rocks as to derive the 

 vertebrate from the invertebrate, or the mammal 

 from the reptile. Of course, it could not be done by 

 a mechanical process alone. It has been largely a 

 chemical process and, no doubt, to a certain extent, 



63 



