2 INTEODUCTOBY 



them, even if we would. We borrow from the rocks that which 

 is essential to onr life to-day, but when that brief day is ended 

 return it once more, with neither loss nor gain, to its original 

 source. 



Those portions of the earth's crust which are available for 

 study comprise at best but a few thousand vertical feet, though 

 from the fact that the stratified rocks have been extensively 

 thrown out of their original, horizontal position, and again 

 eroded, we are enabled to measure their thickness, and may 

 claim to know with a reasonable degree of accuracy the char- 

 acter of the material forming this crust down to a depth of 

 perhaps twenty miles.^ Throughout all this vast thickness, 

 comprising millions upon millions of cubic feet, in weight far 

 beyond all comprehension, is found a constant recurrence of 

 materials alike in composition and similar in origin to those 

 upon the immediate surface. There is at times, as noted later, 

 a diiference in structure, due to metamorphism, between the 

 older, deeper lying portions and those of more recent origin, but 

 the ultimate composition is essentially the same, and all the 

 knowledge thus far gained points to a wonderful unity in na- 

 ture 's methods, and shows with seeming conclusiveness that the 

 geological agencies of the past, the methods by which rocks were 

 made and again destroyed, differed in no essential particular 

 from those in progress to-day. What these processes were, 

 how they operated, and with what results, it shall be our aim 

 here to set forth. 



Among the many interesting, and at first thought seemingly 

 unaccountable, things encountered in the progress of our work, 

 not the least is the fact that so large a proportion of natural 

 objects are more or less out of harmony with their surroundings. 

 Throughout life every organic being is in a constant struggle 

 with the elements to preserve that life, fulfil all its functions,, 

 and gratify its natural desires. No sooner does life depart than 

 decomposition and disintegration ensue. As with organic beings, 

 so with inorganic substances. Every mass of rock pushed up 

 by the faulting and folding of the earth's crust, exposed by 

 denudation, or erupted as molten matter from the earth's in- 

 terior, finds almost at once that its various elements, in their 

 existing combinations, are not in harmony with their environ- 



^ The total mean depth of the f ossilif erous formations of Eu'rope as stated 

 by Oeikie (Text-book of Geology, p. 675) has been set down as 75,000 feet.. 



