44 



University of California. 



[Vol. 2. 



volcanic neck the gabbro forms the cliffs and mountain slope above 

 for fully half a mile. It is in this large area that its coarsest texture 

 appears. Crystals of secondary hornblende reach a length some- 

 times of two inches. The rock mass is not homogeneous in texture, 

 for there are scattered through it, in bunches and irregular veins, 

 coarse aggregates of the main constituents, hornblende and feldspar. 

 Much of the rock has been crushed and in the seams calcite depos- 

 ited. On some rock cliffs the calcite forms a thickly interlacing 

 network of veins. Occasionally cavities appear in the rock, being 

 probably of secondary origin, wholly or partly filled with clear 

 calcite. Dikes varying in character from those which are dark and 

 aphanitic to those distinctly diabasic abound in various parts of the 

 gabbro. Towards the southeastern edge of the gabbro, but fully 

 inclosed within it, is a large body of sheeted rock of aphanitic tex- 

 ture and varying color. Upon the edge of one sheet variolitic and 

 glassy facies occur. 



As the eastern end of the gabbro is approached, it becomes 

 finer and more diabasic in appearance, until, in the cliffs north 

 of the Old Landing, it terminates in long dike-like arms, which 

 extend out into the greatly decomposed aphanitic rocks. It is 

 probable that the gabbro extends underneath the ridge towards 

 the east. The mountain rises above the cliffs, where the gabbro is 

 best exposed, for 1,000 feet very precipitously, and the coarse rock 

 can be traced continuously upward, gradually becoming finer. Near 

 the top there is a sharp transition to the basalt, into which arms of 

 the diabase extend. The canon west of the lower dairy on Corra- 

 litos Creek has cut down through the diabase, as shown by the 

 float. 



Age of the Complex. — The field relations are such that the 

 exact age of these rocks can not be determined. That it is greater 

 than the Miocene is certain. That it antedates the Knoxville is 

 probable, for the following reasons : Its relation to the Knoxville is 

 such as to lead to the supposition that it underlies that formation. 

 The great amount of alteration shown in many places is also indica- 

 tive of age. The spheroidal basalt has a notable resemblance to 

 occurrences through the Coast Ranges which have been described 



