740 



JOHN W. GRUNER 



Of special interest in the outlier are the five members listed as 

 puddingstone conglomerates on account of their unusual texture 

 and composition. Their color as a whole is dark red. Joints and 

 bedding planes are few and far apart in the three lower members. 

 The lowest one (No. 2) of the formation consists of very angular 

 "pebbles" and platy fragments of green chlorite and gray quartz- 

 schist and gray slate, varying in size from mere sand grains to great 

 bowlders 3 to 4 feet in diameter. They are imbedded in a red 



argillaceous and arenaceous cement, 

 which makes up 80 to 85 per cent of 

 the volume of the conglomerate. The 

 highest member of the conglomerates 

 (No. 13) which contains pebbles not 

 exceeding i inch in diameter has only 

 calcite as cement, which contains 

 abundant fossil fragments. 



POST-PALEOZOIC IGNEOUS ROCKS 



Though outcropping in areas 5 to 7 

 miles apart, the Red River rhyolite 

 flow, the intrusive Opal Peak por- 

 phyry, and the numerous rhyolitic 

 dikes are chemically and mineralogi- 

 cally much alike and probably of the 

 same age. They are certainly post- 

 Carboniferous, for one of the dikes 

 cuts the Pennsylvanian beds on the 

 divide east of Red River. 

 Only a part of the thick rhyolite porphyry flow on Red River 

 lies in the area mapped. The rock is hght gray in color. The 

 white porphyry of the extremely rugged Opal Peak and Cuchilla de 

 Media lying between the darker gneisses and granites offers a con- 

 spicuous color contrast. The porphyry in spite of its prominent 

 sheeting (Fig. 4) is very soft, and no dark minerals were seen in it. 

 The scattered white rhyolite porphyry dikes, intrusive into the 

 pre-Cambrian rocks, as a rule have a northwesterly trend and steep 

 or vertical dip. 



Fig. 4. — Opal Peak. Looking 

 northeast. Center of porphyry 

 stock with nearly vertical sheeting. 



