Lawson.] 



Geology of Carmelo Bay. 



13 



and to the ground-mass of the rock on the other, seem to afford 

 some very suggestive and forcible evidence as to the progress of 

 crystallization in the magma which yielded this granite. We can- 

 not regard the large orthoclase phenocrysts, so perfectly developed 

 crystallographically, as the first products of consolidation, which 

 floated about freely in the magma till surrounded by the later con- 

 solidation, in granular form, of the ground-mass. For these ortho- 

 clases are full of crystals which are themselves of the nature of 

 phenocrysts, and which are certainly not of later generation than 

 their hosts. Nor can we regard these inclusions as representative 

 of the first generation of crystals which were subsequently caught 

 up and inclosed by the crystals of a second generation — the ortho- 

 clases, for, as we have seen, the orthoclase exercised a very power- 

 ful control over the distribution and orientation of the inclusions, 

 and this can only be assumed to have been effective during the 

 growth of the latter. The minute idiomorphic crystals so abundant 

 in the phenocrysts are, moreover, not observable in the ground- 

 mass, as they probably would be if they had been the products of 

 the first generation, and owed their inclusion in the orthoclases to 

 their prior free existence in the magma. Thus it appears that these 

 interesting and remarkable inclusions of feldspar and quartz are 

 neither anterior nor posterior to their hosts in generation, but are 

 of simultaneous development. Such a statement cannot at present 

 be made with reference to the inclusions of mica and apatite. 



One other interesting circumstance remains to be noted. The 

 perfection of form of the large phenocrysts of orthoclase has been 

 alluded to. This implies, of course, a sharp demarkation between 

 the bounding planes of the phenocrysts and the ground-mass in 

 which they are imbedded. When the rock has undergone secular 

 decay, the phenocrysts may be picked from the ground-mass in 

 fresh condition and good form. While the phenocrysts have these 

 true crystallographic faces as bounding planes, and are thus sharply 

 marked off from the ground-mass, there is no corresponding sharp 

 demarkation between the inclusions of the phenocrysts and the 

 ground-mass of the rock- And for this reason the surfaces of the 

 phenocrysts, removed from the decayed rock, are always rough, 

 owing to the protrusion beyond the boundary planes of the included 



