98 SCIENCE IN THE LAST HALF CENTUEY. 



world. In geology the study of large maps is as important as it is said 

 to be in politics ; and sections, on a true scale, are even more important, 

 in so far as they are essential to the apprehension of the extraordinary 

 insignificance of geological perturbations in relation to the whole mass 

 of our planet. . It should never be forgotten that what we call "catas- 

 trophes" are, in relation to the earth, changes, the equivalents of which 

 would be well represented by the development of a few pimples, or the 

 scratch of a pin, on a man's head. Vast regions of the earth's surface 

 remain geologically^ unknown; but the area already fairly explored is 

 many times greater than it was in 1837, and in many parts of Europe 

 and the United States the structure of the superficial crust of the earth 

 has been investigated with great minuteness. 



The parallel between Biology and Geology which I have drawn is 

 further illustrated by the modern growth of that branch of the science 

 known as Petrology, which answers to Histology, and has made the 

 microscope as essential an instrument to the geological as to the bio- 

 logical investigator. 



The evidence of the importance of causes now in operation has been 

 wonderfully enlarged by the study of glacial phenomena, by that of 

 earthquakes and volcanoes, and by that of the efiicacy of heat and cold, 

 wind, rain, and rivers as agents of denudation and transport. On the 

 other hand, the exploration of coral reefs and of the deposits now tak- 

 ing place at the bottom of the great oceans has proved that in animal 

 and plant life, we have agents of reconstruction of a potency hitherto 

 unsuspected. 



There is no study better fitted than that of geology to impress upon 

 men of general culture that conviction of the unbroken sequence of the 

 order of natural i)lienoniena throughout the duration of the universe, 

 which is the great, and perhaps the most important, effect of the in- 

 crease of natural knowledge. 



