48(18) MINERAL PHYSIOLOGY. 



build ii]) a fresh portion, which serves to unite the (lis 

 rupted fragments and thus to make the limb whole again- 

 In a like manner does the terrestrial circulation repair 

 the fractures in the earth's crust by the formation of 

 veinstones ; an example in the mineral world of nature's 

 conservative surgery. 



There is another side to the process of growth and 

 production of new rock-masses which we have just con- 

 sidered. The new is built up by the decay of the old, 

 and it would be instructive to study at length the pro- 

 cesses of disintegration and solution which are the neces- 

 sary preliminaries to reproduction and palingenesis. There 

 is however one phase of tin;; destructive metamorphosis 

 of mineral matter upon which I may be permitted to 

 dwell for a few minutes, — that of the subaerial decay 

 of rocks, or their alterations when exposed to the 

 influences of air and moisture. Gfoing back once more 

 in imagination to the time when there were no rocks 

 newer than our ancient Laurentian gneisses, with their 

 accompanying interstratilied limestones and iron-ores 

 we discover that there were then no sandstones, no clay- 

 slates, nor in fact anything to represent the vast de- 

 posits of uncrystalline sands and clays of later times. All 

 of these are results of a subsequent process of subaerial 

 decay. Slowly but inevitably the solid granites, gneisses 

 and similar crystalline rocks decay like dead trees in a 

 forest. Water and air conjoined cause the breaking up 

 of the feldspars, the hornblendes, and many other min- 

 eral silicates which enter into the composition of these 

 rocks, and thus convert these into a crumbling earthy 

 mass. Much of the silica, besides the lime, the mag- 

 nesia, the potash, and the soda of these minerals, passes 

 into solution, and little more than clay, iron-oxyd and 

 quartz, remain behind in the softened and decayed mass, 

 which has lost one half or more of its weight, and now 

 yields readily to the mechanical action of water, by 

 which it is separated into clay on the one hand and sand 

 on the other. Geological investigations show that such 

 a process of decay has gone on continuously from the 

 Laurentian age to the present, whenever crystalline rocks 

 have been exposed to the action of the elements, and that 

 this process has been the source of all the clay and 

 most of the sand which enter so largely into the com- 



