276 University of California. [Vol. i. 



induced is very similar in the series of dikes near the Oceanic mine. 

 Microscopically but little of interest appears in any of the slides 

 prepared from the metamorphosed contact rocks. Mica, in the 

 form of minute scales, seems to be the most important secondary 

 mineral which can with certainty be made out. A recrystallization 

 of the calcite has taken place in the sandstone forming individuals 

 inclosing the other components, as shown by the luster on fresh 

 surfaces of the rock. 



MACROSCQPICAL CHARACTER OF THE CUYAMAS OCCURRENCE. 



This eruptive mass consists of two distinct portions: (1) The 

 main one, which is dark and fine grained; (2) ramifying dikes of 

 later origin, much harder and lighter colored. The main body has 

 the appearance of being very greatly decomposed. It is with diffi- 

 culty, even in the deepest canons, that hand specimens can be 

 obtained which will not crumble upon a slight pressure. The mass 

 is entirely free from any schistose tendency, but in places inclines 

 to a shelly spheroidal structure. Such is the ease with which the 

 rock disintegrates that the contour of the hills which it forms is not 

 noticeably different from that of the unconsolidated Tertiary strata 

 of a large part of the range. 



The dikes which intersect the main body form the most inter- 

 esting feature of the whole occurrence. They have a somewhat 

 similar composition, but are generally much fresher. They cut the 

 main dike in all directions, and are in most cases rather narrow, 

 ranging from about six inches to two feet in width. They are more 

 numerous near the western edge of the main mass, where there are 

 several ten to eighteen feet wide. The most of these are hard and 

 project above the crumbling rock inclosing them. As the latter 

 weathers away, the dikes break up into masses of varying size and 

 roll into the canons. Occasionally one of these dikes is almost as 

 dark and decomposed as the inclosing mass, but generally they are 

 in sharp contrast with the latter. The very narrow ones are finer 

 grained than the inclosing rock, but the broad ones are considera- 

 bly coarser. All possess a granular structure which is as pro- 

 nounced in the narrow ones as in the broad ones. It is probable 

 that after the cooling of the upper portion of the main mass a strain 



