2 CALIFORNIA ACADEMY OF SCIENCES. 



I. — Granite and Diorite. 



It has been shown in the paper referred to that the struc- 

 ture of the peninsula at the latitude of Ensenada is that of a 

 monoclinal mountain range with a gentle slope towards the 

 Pacific and an abrupt faultscarp, facing the Colorado Des- 

 ert. Furthermore, that the range is chittiy made up of 

 a coarse-granular granitic rock, in which are intercalated 

 several smaller areas of more or less altered slates, dipping 

 towards the east at a high angle. Volcanic flows of differ- 

 ent age are spread over the long western slope, and are 

 especially accumulated in heavy masses near the Pacific. 

 Again, at the foot of the faultscarp, near the western limit 

 of the Colorado Desert, eflusive rocks occur, the age of 

 which probably is pliocene, or recent. 



In the paper quoted, the granitic rock has several times 

 been described as a hornblende granitite (Rosenbusch), 

 being a coarse granular mixture of biotite, hornblende, feld- 

 spar and quartz. From the examination in the field and 

 the great resemblance to the granitic masses of Southern 

 and Central California, there seemed to be but little doubt 

 that this would be the correct classification. 



The detailed petrographical examination of specimens 

 from very different localities along the section has shown, 

 however, that the predominent feldspar usually is a plagio- 

 clase, and that the orthoclase only occurs in comparatively 

 small quantities, or may even be entirely absent. 



While the quantity of hornblende and biotite maj vary 

 within certain limits, the general habit and appearance of 

 the granitic rock is remarkably constant from Ensenada to 

 the desert. It is rather coarse-grained and easily disinte- 

 grating, giving to the bluffs the well known ' ' woolsack- 

 structure." When decomposing, it often assumes a yellow- 

 ish color, due no doubt to the large proportion of ferromag- 

 nesian silicates which it contains. 



