Lawson.) 



Geology of Carmelo Bay. 



'5 



diorites. The ground-mass of the rock certainly belongs to that 

 intermediate type for which Becker has recently proposed the term 

 granodiorite* Pyritc is occasionally present. 



A feature of interest connected with the physical character of the 

 ground-mass is the abundance of minute cracks which traverse it 

 in all directions. These cracks may be readily observed with the 

 aid of a lens. They cause the rock to be rather friable and easily 

 susceptible to disintegration. It is very difficult on their account 

 to make satisfactory slides of the ground-mass. These cracks do 

 not appear to obey any definite law of direction, and may, perhaps, 

 be ascribed to the presence of the large phenocrysts, and the un- 

 equal and intermittent tension set up by their differential expansion 

 and contraction in different crystallographic directions. It is, how- 

 ever, known that the rock has been subjected to very considerable 

 mechanical stresses of an orogenic character since the deposition 

 upon it of newer formations, and the possibility of the irregular As- 

 suring being due to these strains must be considered. If the latter 

 were the cause, we would expect to find definite systems of shear- 

 ing and faulting, and the phenocrysts should also be affected. The 

 phenocrysts are, practically, unaffected by these cracks, so far as 

 has been observed. The ready disintegration induced by these 

 fissures, together with the occasional occurrence of pyrite in the 

 rock, are probably the causes which have led to the abandonment 

 of the quarry as a source of building material. 



The following is an analysis of the rock from material selected 

 so as to represent as nearly as possible the composition of the gran- 

 ite as a whole, a selection requiring some judgment on account of 

 the large size of the phenocrysts: 



*Geological Atlas of the U. S., Sacramento sheet. Glossary of rock names. 



