154 CALIFORNIA ACADEMY OF SCIENCES [Proc. 4th Se«. 



Solecurtus cumming-ianus Dunk. 



" gibbus Spengl. . 

 Semele perlamellosa Heilpr. 

 Panopaea floridana Heilpr. 

 Xenophora conchyliophora Born. 

 Sigaretus cfr. multiplicatus Dall. 

 Turritella Aguilerae Bose. 

 Cerithium caloosaense Dall. 

 Strombus pugilis Linnaeus (frequent) 

 Pyrula papyratia Say (frequent) 

 Dolium cfr. galea Linnaeus 

 Oliva litterata Lam. (frequent) 

 Balanus eburneus Gould. 



Nearly all the fossils occur in the form of casts and it is 

 not possible to determine a number of the species on account of 

 the absence of ornamentation or because, being new species, 

 they cannot be determined for want of better preserved ma- 

 terial. 



This fauna appears to be a littoral or at least comparatively 

 shallow water one and many of the species or their kindred are 

 still living in the adjacent sea. The Santa Maria Tatetla, 

 Santa Rosa, and Tuxtepec species appear to belong to the same 

 fauna and same age, which, although given as Pliocene, in the 

 publication quoted, is now regarded (so Bose says) on account 

 of larger and more complete collections, as Miocene. 



Igneous Rocks 



The igneous rocks occurring in this area are nearly all 

 basaltic. 



They occur as dikes of various widths, as plugs or bosses, 

 and in beds forming the tops of hills and mesas. 



The great number and extent of the dikes suggest that the 

 lavas which form the caps of the hills and mesas came up as 

 sheet flows rather than through craters. 



Huntley'* gives two maps showing the location of a number 

 of these dikes running in different directions, together with sur- 

 face flows and states that the peaks and plugs of basalt are 

 usually found at the intersection of such dikes. 



" Trans. A. I. M. E. LII. pp. 302. 310. 



