IMPORTANCE OF GASES IX CRYSTALLIZATION. 265 



secondary origin. Rosenbusch, in his " Microscopic Physiography of 

 the Massive Rocks " and in his polemic with Michel-Levy, has often 

 referred to the importance of water as a mineralizer. 



Iddings * has brought forward very clearly the possibility of rocks of 

 very similar chemical composition having widely different mineral com- 

 position, and exjDressly assigns mineralizing agents as one cause of the 

 difference. If I understand him and Dr Wadsworth also correctly, they 

 seem to rank rate of cooling, so far as primary textures are concerned, as 

 more important than the mineralizers. That rate of cooling is most im- 

 portant in determining the size of grain we shall all agree, but not so as 

 to the replacement of minerals by others, except in the case of replace- 

 ment by glass. I have been recently much struck, in studying certain 

 massive flows of mottled melaphyre on Isle Royale, with the gradual in- 

 crease in coarseness of the mottling, which is due to the increase in size 

 of the patches of augite which enclose the felspar laths as we pass from 

 the margin ; but, besides these evidences of slower rate of cooling, we 

 find to a large degree independently, in thin intrusive sheets, or in spots - 

 or in streaks, a very different texture — a more distinctly plutonic gabbro 

 texture. This suggests another line of facts in the textures in the pro- 

 duction of which minerals have been essential. The writings of Brogger, 

 Rosenbusch, Irving, Romberg f and others on micropegmatite and peg- 

 matite, and of Cross and Iddings, Rutley and others on spherulites, are 

 full of hints in this direction. The miarolitic and panidiomorphic tex- 

 tures are other textures characteristic of the presence of mineralizers. 

 On the other hand, the glassy, hyalopilitic, vesicular and am3^gdaloidal 

 textures are characteristic of the escape of mineralizers. Iddings, how- 

 ever, argues that as these mineralizers are present in all magmas the dif- 

 ferences in textures and mineral composition are to be credited rather to 

 the other geologic conditions than to them. But what if the other con- 

 ditions are those which permit the rapid escape of the mineralizers or 

 retain them practically throughout crystallization? Would it then be 

 an abuse of langu<age to say that the mineralizers were an essential factor 

 in separation of volcanic and plutonic rocks ? The diverse character of 

 the contact-zones, which will ])e discussed a little later, also points to the 

 escape of the mineralizers as the important distinctive character of vol- 

 canic from plutonic rocks. 



Deep-seated Cracking. 



Its menial Conception difficult. — We pass now to the second suggestion, to 

 my mind by far the most shaky portion of the fabric. Can cracks ex- 



* An. Rep. U. S. Geol. Survey, 1891, p. 657; also Proc. Phil. Soc. Washington, 1890, p. 213. 

 tNeues Jahrl)nch, 1892; Beilage Band, vol. viii, p. 300. 



