. Smithsonian miscellaneous collections . the Mexican Cambrian is divided. Afterthe description of this section the sequences at other localities will beexplained. LOCALITIES Proveedora Hills (Cerros de la Proveedora) (Locality 801) The Cerros de la Proveedora are a roughly longitudinally oval groupof hills trending northward and located about 6 miles west of Caborcaon the north side of the road through the gap known as Puerto Blanco(text figs. I, 2, 5). The main mass of the hills consists of two elongatedridges separated by a deep valley interrupted in its middle by a lowsaddle. These hills l
. Smithsonian miscellaneous collections . the Mexican Cambrian is divided. Afterthe description of this section the sequences at other localities will beexplained. LOCALITIES Proveedora Hills (Cerros de la Proveedora) (Locality 801) The Cerros de la Proveedora are a roughly longitudinally oval groupof hills trending northward and located about 6 miles west of Caborcaon the north side of the road through the gap known as Puerto Blanco(text figs. I, 2, 5). The main mass of the hills consists of two elongatedridges separated by a deep valley interrupted in its middle by a lowsaddle. These hills lie on the north side of a large granitic mass whichhas metamorphosed their southern edge and, in one place at least, in-truded the Cambrian beds. The southern end of the hills is brokeninto several knobs of various sizes, mostly composed of white marble,clearly an alteration product of contact metamorphism. The Proveedora Hills are composed wholly of Cambrian sedimentsbut neither the stratigraphic top nor bottom of the sequence is 4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. II9 The lowest sediments appear in the small knob on the west end of thehills. The strike of the sediments is a few degrees west of north(N. 7° W.) and the dip is quite variable, but all the angles measuredare steep, generally greater than 60°. The dip in the west ridge and thehills to the west is generally to the east. Dips on the west side of thesaddle between the two main ridges are to the west, but on the oppositeside of the saddle they are to the east. Strong lithological differencesin parts of the hills permit division into six formations, in ascendingorder: Puerto Blanco, Proveedora, Buelna, Cerro Prieto, Arrojos,and Tren formations (text figs. 4, 5). The fossil content of the firstthree indicates a Lower Cambrian age. The Cerro Prieto formationhas not been dated, but the Arrojos and Tren formations belong inthe Middle Cambrian. Puerto Blanco formation.—This formation occupies the low rid
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