246 PROCEEDINGS OF THE CALIFORNIA 



enabled to obtain a pretty good general idea of the geology of that region ; 

 and, in Sonora, to which my attention was especially devoted, I succeeded in 

 finding fossils in sufficient quantity to allow of the determination of the age of 

 the principal formations of the northern Sierra Madre. By tracing the connec- 

 tion of these rocks with those of Central Mexico, additional light will be thrown 

 on those districts of which, at present, but little is definitely known. 



The igneous rocks, which occur more abundantly on the Pacific slope, are 

 granites, either fine or very coarse-grained ; porphyries, more or less feidspathic ; 

 and greenstones, all of which are cut by numerous dykes of extremely varied 

 character. The granites, however, are very poor in veins of the precious metals, 

 while the porphyries are highly metalliferous. In Sinaloa (Candelero) and 

 Durango (San Dimas) we see that the granites underlie the metalliferous por- 

 phyries, and that the greenstones, in Sonora, (near Hermosillo and in the vicin- 

 ty of La Haciendita) penetrate through them. 



The oldest sedimentary rocks, which I have observed, belong to the Carbon- 

 iferous series ; this is represented in the eastern part of Sonora, by heavy masses 

 of limestone, forming very high and rugged ridges, running a little west of north. 

 The upturned strata are seen, in many places, to rest on granite. Argentiferous 

 veins occur throughout this formation. 



The next group of sedimentary rocks, in order, is the Triassic; this forms 

 isolated mountain groups in Sonora, and offers an interesting field for investiga- 

 tion. Instead of limestones, it is made up of heavy beds of quartzites and 

 conglomerates, with coal-bearing clay shales ; all of these are disturbed and 

 elevated, and rest on greenstones, feidspathic porphyries, or granite. "Wherever 

 metamorphosed, the Triassic rocks are auriferous and contain veins of silver ores. 

 The metamorphic slates and limestones of the Altar and Magdalena districts, 

 which include the richest gold placers of Sonora, may possibly be of Triassic 

 age ; but the fossils collected are too imperfect to admit of this being determined. 

 There are some reasons for believing those rocks to be rather of Jurassic than 

 of Triassic age, as they differ in lithological characters from both the Triassic 

 and Carboniferous of Northern Mexico, resembling, rather, the Jurassic gold- 

 bearing slates of the Sierra Nevada, in California ; besides, they lie outside and 

 to the west of the Sierra Madre. It may also be noticed that the gold which 

 they furnish does not resemble that obtained from the Triassic strata. 



The Cretaceous period is also represented at the foot of the Sierra Madre, at 

 Arivechi, in Sonora. The strata belonging to this series are chiefly argillaceous 

 shales, and they rest upon porphyries and Carboniferous limestone. They have 

 been disturbed and elevated since their deposition. The fossils, which they con- 

 tain in great number and in a fine state of preservation, will be noticed farther on. 

 All the above mentioned formations were already in existence before the first 

 eruption of the volcanic rocks took place. These latter are found scattered 

 along the whole Pacific coast, and extend from the Gulf of California up to the 

 very summit of the sierra. It is very interesting to see the volcanic formations 

 spread over so extensive a region, especially as there are no active volcanoes 

 known in Northern Mexico, and not even any indications of ancient craters or 

 vents. 



