256 FINLAY 



the limestone cover. It is also later than the exposures of 

 diorite indicated on the map along the road leading to the Ve- 

 gonia Mine, and in the stream-way a mile to the northeast of 

 San Jose. The andesite has sent numerous small dikes into the 

 diorite. This rock is met in three rounded bosses, enclosed by 

 the andesite, and much weathered. It is often friable, and 

 readily breaks up into a rusty brown soil like that which is fur- 

 nished by the weathering of diabase. 



On the eastern slope of the San Carlos range, and five miles 

 from San Jose to the south, there is met a lava flow of basalt. 

 The upper surface of the lava field, which is six miles long and 

 a mile in width, is rough and vesicular. Much has been car- 

 ried away from its surface by erosion. The body of the rock 

 in the field is dense and aphanitic, rarely glassy. The ba- 

 salt is black, at times with a slightly vitreous luster. It is of 

 course much more recent than the massive nephelite syenite 

 down whose flanks it flowed, and it is presumably younger than 

 the diorite and the dike rocks. None of these were found cut- 

 ting it. No cinder cone could be located in connection with it, 

 if one ever existed. 



The dikes, which are very numerous throughout the dis- 

 trict, are of tinguaite, camptonite, diabase and vogesite. They 

 vary in width from half an inch to ten or twelve feet. They 

 are usually found but slightly inclined away from the vertical. 



The tinguaites are distinct in field habit from all the others. 

 They are uniformly colored green. Two types are found 

 among them — those which carry conspicuous phenocrysts of 

 sanidine, and those which are aphanitic, and are characterized 

 by showing no porphyritic developments in the hand specimen. 

 No tinguaite dikes were found cutting the nephelite syenite. 

 Where they lie in the limestone they are seen to have had 

 almost no effect upon it. The blue sedimentary rock continues 

 up to the contact without being changed in any way. Two 

 very large tinguaite dikes are indicated on the map in the 

 midst of the andesite. The longer of them runs for two and a 

 half miles. It carries phenocrysts of sanidine, by which its 

 even clear green color is mottled with white. Its outcrop 



