84 JOURNAL OF THE 



number of elements under the circumstances of heat and pressure 

 in a liquid mass from which mica, feldspar and quartz were after- 

 wards to crystallize might well be different from what would 

 obtain if the limiting circumstances were withdrawn. Potash, 

 alumina and silica do not combine at ordinary temperatures, nor 

 do lime, iron and silica. 



Whatever the affinities between these substances might have 

 been before crystallization, when this process was once established 

 it w 7 ent on to form perfectly definite compounds. Which one 

 crystallized first is not so quickly said. From evidence now in 

 my possession I am inclined to believe that the mica crystallized 

 first. I was led to this conclusion, not by theoretical considera- 

 tions, but by having found in a mica vein a piece of quartz hav- 

 ing on it evident impression of the edges of a block of mica, 

 forming a sort of pyramid with microscopic steps; iuclosures of 

 quartz between the lamina? of mica, the quartz being almost as 

 thin as the mica; iuclosures of feldspar in mica also very thiu, 

 and lying pressed between the mica sheets. 



These three circumstances taken in conjunction would seem to 

 indicate a crystallization of the mica prior to that of the quartz 

 or feldspar. A synchronous crystallization would have given a 

 mass more nearly resembling granite, in which each substance 

 has interfered with the other. So far then as the moment of 

 crystallization is concerned, a mica vein differs from granite in 

 having suffered a succession of crystallizations instead of syn- 

 chronous crystallization. Had the mica, feldspar and quartz all 

 crystallized at the same time, there is no reason why there should 

 not have been granite in the fissures instead of a mica vein. 



It will appear from the preceding discussion that a mica vein 

 is only a vein of very coarse granite in which the forces of crys- 

 tallization have had comparatively free play. The resulting 

 crystals are of great size, and have interfered but little in each 

 other's development. So far as the texture of the vein is con- 

 cerned it is as different from that of ordinary granite as a collec- 

 tion of single crystals of large size is from an agglomeration of 

 crystals of small size. The almost exclusive occurrence of well 



