12 CALIFORNIA ACADEMY OF SCIENCES. 



Porpliyrite Tufa . 



Connected with these flows of quartz-porphyrite there are 

 considerable masses of tufaceous character; sometimes, as 

 near Hotel Iturbide at Ensenada, the tufaceous character is 

 well preserved, although the rock is exceedingly compact, 

 and to a great extent, filled with secondary minerals, epi- 

 dote, chlorite, etc.; often, however, the tufas can only, un- 

 der the microscope, be distinguished from the massive por- 

 j)hy rites. All of them belong to the agglomeratic tufas 

 (Llosenbusch), consisting of small fragments of the primary 

 rock cemented together by a fine detrital mass. 



The tufa from near Hotel Iturbide is a dark rock with a 

 flinty fracture and many small feldspar crystals. The clastic 

 character is most clearly perceived on the weathered faces, 

 which distinctly show the different fragments in relief. 



Under the microscope the angular fragments consist of 

 quartz porphyrites with holocrystalline groundmass, more 

 or less fine, containing much opacite, and usually to be re- 

 ferred to the type described above under 1. Besides, there 

 are numerous fragments of porphyritic feldspar and quartz. 



All of these are cemented by a very tine microcrystalline 

 groundmass, evidently once detrital, but in which now this 

 character has been obscured by secondary processes. Other 

 tufas contain, besides the fragments of porphyrites, granitic 

 quartz and feldspar. 



III. — Neovolcanic Effusive Kock. 



Basalt. 



The only recent or tertiary volcanic rocks examined were 

 those from the flow, coming down from some point in the 

 interior and ending at the steep bluffs on the coast north of 

 Todos Santos Bay. 



Specimens collected near Sausal have the macroscopic 

 appearance of a black, fine-grained vesicular basalt; the 

 cavities are usually covered with a yellow coating. 



