580 THE POPULAR SCIENCE MONTHLY. 



a clew to what happens when water freezes and .melts or when it is 

 turned into steam ! It is only within fifty years that physics and 

 chemistry have begun to assume the form of coherent bodies of 

 scientific truth. Evidently geology could not be expected to take 

 scientific shape until late in the eighteenth century, or to make any 

 notable conquests before the nineteenth. But when geology did 

 win its first great triumph, about sixty years ago, it was in some 

 ways the most remarkable moment in the history of thought since 

 the promulgation of the Newtonian astronomy. Newton proved 

 that the forces which keep the planets in their orbits are not 

 strange or supernatural forces, but just such forces as we are fa- 

 miliar with on this earth every moment of our lives. Geologists 

 before Lyell had been led to the conclusion that the general aspect 

 of the earth's surface with which we are familiar is by no means 

 its primitive or its permanent aspect, but that there has been a suc- 

 cession of ages in which the relations of land and water, of mount- 

 ain and plain have varied to a very considerable extent, in which 

 soils and climates have undergone most complicated vicissitudes, 

 and in which the earth's vegetable products and its animal popu- 

 lations have again and again assumed new forms while the old 

 forms have passed away. In order to account for such wholesale 

 changes, geologists were at first disposed to imagine violent catas- 

 trophes brought about by strange agencies agencies which were 

 perhaps not exactly supernatural, but in some unspecified way 

 different from the agencies that are now at work in the visible 

 and familiar order of Nature. But Lyell proved that the very 

 same kind of physical processes which are now going on about 

 us would suffice during a long period of time to produce the 

 changes in the inorganic world which distinguish one geological 

 period from another. Here, in Lyell's geological investigations, 

 there was for the first time due attention paid to the immense im- 

 portance of the prolonged and cumulative action of slight and un- 

 obtrusive causes. The continual dropping that wears away stones 

 might have served as a text for the whole series of beautiful re- 

 searches of which he first summed up the results in 1830. As 

 astronomy was steadily advancing toward the proof that in the 

 remotest abysses of space the physical forces at work are the same 

 as terrestrial forces ; so now geology, in carrying us back to enor- 

 mously remote periods of time, began to teach that the forces at 

 work have all along been the same forces that are at work now. 

 In that early stage when the earth's crust was in process of forma- 

 tion, when the temperature was excessively high, there were, of 

 course, phenomena such as can not now be witnessed here, but to 

 find a parallel to which we must look to certain other planets 

 such as violent atmospheric disturbances, and such as the disso- 

 ciation of chemical elements which we are accustomed to find in 



