THE GEOLOGY OF MEXICO. 6ii 



lite, pegmatite, etc. Since no fossiliferous beds are known 

 of earlier date than Carboniferous, it is evident that the 

 age of these crystalline and schistose rocks is at best very 

 uncertain. 



PALEOZOIC. 



Silurian. — In the collection of the Institute there is a 

 fragment of limestone displaying very fine specimens of 

 Ortliis testudinariay Dalm ; and this is stated to come from 

 La Cuesta de Santa Teresa, one league (about two miles 

 five furlongs) east of Cacahuamilpu, in the state of Guerrero. 

 Several expeditions, however, have been sent, without suc- 

 cess, to discover these deposits, and the occurrence must 

 be considered doubtful. 



Other rocks of various ages have been placed in the 

 Silurian by various writers, but the correlation is in no case 

 based on satisfactory palaeontological evidence. 



Devoniaji. — In the same collection there is a fine ex- 

 ample of a Goniatite, which is referred to the Middle 

 Devonian. The specimen is stated to have come from 

 the ridge of Apulco ; but here again subsequent expedi- 

 tions have failed to discover any further specimens. It is 

 believed, however, that certain schists below the Jurassic 

 rocks of this neighbourhood may have yielded this fossil, 

 and may really be of Devonian age. 



Carboniferous. — The occurrence of Carboniferous rocks 

 in Mexico has been definitely proved. They are found 

 on the borders of Guatemala, where they contain Pro- 

 ductus semireticulatus. The rocks are limestones and 

 dolomites, passing down into sandy conglomerates. 



Dr. Persifor Frazer has referred to the Upper Carboni- 

 ferous the limestones which form some of the ranges in the 

 states of Coahuila, and Nuevo Leon ; and he bases this view 

 on the identification of a certain number of fossils by James 

 Hall and A. Heilprin. But the specimens are too badly 

 preserved to be specifically identified, and even the generic 

 determinations are open to doubt. From the results of 

 other expeditions to these localities, the limestones are now 

 believed to be the Cretaceous. 



