ACADEMY OP NATURAL SCIENCES. 249 



The greenstones or diorites which occur in the granite, appear to be anterior to 

 the metalliferous greenstones, and the latter are posterior to the Triassic. 



Carboniferous Limestones. 



The Carboniferous limestones form high ridges parallel with the general 

 course of the Sierra Madre, from Hermosillo, north of Guaymas, east of Sa- 

 huaripa. These ridges become more elevated as we approach the crest of the 

 Sierra. The rocks of this formation are fine-grained and bluish in color, and 

 form heavy beds with intercalated schistose layers ; they contain nodules and 

 beds of flint. There are some clay slates at the base of the formation. The 

 thickness of the whole series is probably over five thousand feet. The principal 

 localities where these Carboniferous rocks may be observed are as follows, nam- 

 ing them in order from west to east : 



1st. Hermosillo, where they rest on syenitic granite and are highly metamor- 

 phosed, the limestones being converted into white saccbaroidal marble, and the 

 slates into garnet and epidote rock. Dykes of green porphyry cut through the 

 beds of sedimentary rock, which beds have a strike of about N. 65° W., and 

 stand nearly vertical. 



2d. Five leagues from Hermosillo, at La Cruz ; in the Cerro de Santa Teresa 

 on the south, and the Sierra de Las Auimas ou the north. Here the limestones 

 contain crinoids. 



3d. Four leagues farther on, between La Noria and El Aguajito; here are 

 high granite ridges with a granite axis. 



4th. Twenty leagues from Hermosillo, south of Ures ; Carboniferous rocks 

 upheaved on the southwest side of the granitic Sierra de Mazatan. The di- 

 rection of this range is from northwest to southeast, and its height sixteen 

 hundred varas, according to M. De Fleury ; here are a few silver mines. 



5th. Haciendita, nine leagues farther northeast. The beds here are meta- 

 morphosed and much disturbed, dipping northeast ; these outcrops form low hills. 



6th. Between Matape and Batuco ; a very high ridge of granite, running in 

 a northerly direction, with limestone resting upon it. To the north and east of 

 Topisco the limestones attain a great thickness and afford fine fossils. At the 

 Cerro de la Bonacina, one of the highest points of the range, a variety of 

 corals, crinoids, and brachiopods may be seen weathered out from the surface of 

 several beds of hard, compact limestone, of various colors ; these beds are near 

 the summit of the mountain. This locality was first discovered by Don An- 

 tonio Moreno, Engineer of the Bronces mine. The strata here are much dis- 

 turbed, and appear to have been folded into a mass with a synclinal structure.* 



Triassic Bocks. 

 This formation is usually highly metamorphosed, and passes into porphyries 



* Only a few specimens of the Carboniferous fossils collected by M. Keuiond have ever 

 been received, owing to circumstances connected with the present political condition of 

 Mexico. It is hoped, however, that they are not lost, and that they may yet be recovered. 

 Among the few specimens received is a coral, not to be distinguished from the Lithostrotion 

 (L. mamillare) found near Bass's Ranch, in Shasta County, California. j. d. w. 



