SOME MODERN ASPECTS OF GEOLOGY. 647 



We encounter, in thin sections of both volcanic and metamor- 

 phic rocks, microscopic crystals arrested in every stage of their 

 growth, and it is not true that these earlier forms are mere epit- 

 omes of the perfected individual. We have the fundamental 

 globulite and the complicated and fantastic " growth - forms," 

 which are as different from the finished crystal as is the larva 

 from the butterfly. Thus, to one familiar with such facts as these, 

 there can be no confusion in speaking of the " embryology of a 

 crystal." We think with wonder of the marvelous vitality of 

 seeds which sprouted after three thousand years spent in Egyptian 

 pyramids, and yet the " vitality " of a crystal is such that it will 

 continue its growth under favorable conditions after any number 

 of thousands of years of interruption. 



There is, however, nothing among the recent disclosures of the 

 microscope in regard to rocks so surprising as their delicate ad- 

 justment to their environment. We are accustomed to look upon 

 the masses of our mountains as the very type of what is sta- 

 tionary and eternal ; but in reality they are vast chemical labora- 

 tories full of activity and constant change. With every altera- 

 tion of external conditions or environment, what was a state of 

 stable equilibrium for atoms or molecules ceases to be so. Old 

 unions are ever being broken down and new ones formed. Life in 

 our planet, like life in ourselves, rests fundamentally on chemical 

 action. The vital fluid circulates unceasingly through the arteries 

 of the oceans and the currents of the air ; it penetrates the rocks 

 through the finest fissures and invisible cracks, as the human 

 blood penetrates the tissues between artery and vein, producing, 

 with the help of heat and pressure, like changes in the histology 

 of the globe. The recurrence, after a long interval, of the same 

 set of conditions in the same rock-mass, may bring about the un- 

 ending cycle — analogous to succeeding generations — which Hut- 

 ton, the earliest of the Scotch geologists, recognized a hundred 

 years ago. 



Such processes as these, which properly represent the physi- 

 ology of our earth's crust, have long been suspected, but their 

 exact nature and details are only now being gradually disclosed 

 by microscopical studies of the rocks. 



Suppose, for instance, that a lava-stream bursts from the side 

 of some volcano. As it flows onward, quickly solidifying and 

 crystallizing under circumstances of intense heat, chemical com- 

 pounds are produced which accord with such conditions, but per- 

 haps not with those ordinarily obtaining at the earth's surface. 

 If this is the case, the hardened lava will be in a chemically un- 

 stable state, and will tend in turn to adapt itself to its new sur- 

 roundings by chemical change. 



Countless examples of this adaptability of rocks to their envi- 



