128 



upheaval is of very recent date, Is proven by the presence of exist- 

 ing species of marine shells scattered over the surface. 



As I have already said, the majority of the mountain ranges are 

 granitic, but we find in many places, and especially those where 

 the parallelism is disturbed, extensive representations of other form- 

 ations. 



Usually, outcroppinsrs of gneiss, micaceous, talcose and clay slates 

 are observable, underlying the quaternary at the base of the gran- 

 itic ranges. In many places the plains consist entirely of the detri- 

 tus of these rocks, showing that they extend from mountain to 

 mountain. Towards the Gulf of California, these slates are accom- 

 panied by metamorphic limestone, and often appear forming inde- 

 pendent ridges, or inclined against the higher granite hills. They 

 form the gold region of Sonora, and are probably of the same age 

 as the similar formations of California, of which, indeed, they seem 

 to be the continuation. We find them rising out of the desert, at 

 intervals, from Sonora to the Gila river and the Colorado, and again 

 underlying the tertiary on the western skirt of the Colorado desert, 

 and at various points in Southern California. 



Near the coast, and traversed by the boundary line, is a very 

 interesting volcanic formation. The country is studded over with 

 volcanic cones, some containing craters ; immense streams of lava 

 cross the desert, or cover, as with a mantle, high granite hills. 



The next formation of importance is that of the stratified con- 

 glomerates. These occur in strata of very variable thickness and 

 texture, but all are composed for the most part of fragments of 

 quartziferous porphyry, cemented by a feldspathic mass, also quartz- 

 iferous. This formation is traversed by intrusive dykes of a por- 

 phyry of a similar character to many of the fragments enclosed in 

 the conglomerates. 



There is also a great variety of porphyries, both quartziferous 

 and free from quartz ; and these are the rocks which for the most 

 part stand in the closest connection with the veins of the country. 



Many of these porphyries appear to be the result of metamor- 

 phic action on sedimentary rocks; but others have every character- 

 istic of an eruptive origin. 



Lastly, dykes of a trachytic porphyry, and of a cellular, black 

 rock usually in connection with a reddish icacke and a sandstone, 

 are observable at various points through the countiy. 



Climatic influences have given the country a marked and peculiar 

 character of vegetation. Towards the coast, the plains are barren 

 and arid deserts, and the traveler may ride hundreds of miles with- 

 out seeing other plants than diy and thorny cacti and scattered 

 l)ushes of greasewood. The granite mountains bordering these 

 deserts are even more barren. Not a tree, nor even a cactus, can 



