560 BULLETIN UNITED STATES GEOLOGICAL SURVEY, 



metamorphosed sediuientaries. The latter are so chauged in places that 

 their origiual character is entirely lost, and it is only by tracing them 

 continuously from points where they are unchanged that tliey can be rec- 

 ognized as sedimentary. There are, perhaps, two areas of the porphyritic 

 igneous rock, and they were once doubtless connected. From the central 

 nuclei there are radiating dikes aud intruded sheets. On the north, 

 Carboniferous rocks prevail between the La Phita Mountains and the San 

 Miguel Mountains. Carboniferous also prevails to the east, while on the 

 west and southwest the Cretaceous rocks are in contact with the igneous 

 rock. On the south, the Cretaceous aud underlying formations, as far as 

 the Triassic, are seen dipping away from the mountains. In the region of 

 the La Plata Mountains, there is a general dip of the stratified beds to 

 the westward. In the immediate vicinity of the mountains, there is a 

 local dipi)ing away from the eruptive centre or centres. The rock of 

 the La Plata area resembles that of the areas already described. 



EL LATE AREA. 



The El Late area is 32 miles west of south from the La Plata Mount- 

 ains and about 48 miles east of south from the Sierra Abajo. It is com- 

 posed of trachyte of considerable variety in color and texture, being 

 light and dark, fine-grained and coarse. There is a general uplift of 

 the sedimentaries surrounding the group of mountains, aud immediately 

 at the base the edges of the strata are sharply upturned, except at the 

 north, where the sandstones of the Dakota group pass under the erupted 

 material. There appears to have been a spreading of the trachyte here 

 at the time of its pouring out. The area ot trachyte is somewhat oval- 

 shaped, being 4 miles long and about 1| miles in width, with a north and 

 south axis. There is one principal peak — Ute Peak — at the northern end 

 of the area. On the east side, the Cretaceous shales prevail, extending 

 a short distance around the northern and southern ends. On the west, 

 the Dakota group outcrops, with the underl;\iug variegated shales ap- 

 pearing as we go south. There are included masses ot the Cretaceous 

 shales, distorted and metamorphosed, in all parts of the trachyte. The 

 erupted material appears to have been forced out through fissures, 

 causing a general uplift, and ou reaching the Cretaceous shales it spread 

 laterally. How far it reached vertically, we cannot now tell. There is 

 but one point — the Ute Peak — where there is sufficient thickness to have 

 penetrated the sandstones of the Upper Cretaceous; but we must re- 

 member that the erosion subsequent to the eruption affected not only 

 the sedimentaries but also the trachyte itself. At the north is a dike 

 extending northward from the main mass. 



CARRISO AREA. 



The Carriso is another of the groups studied by Mr. Holmes. It is 

 32 miles, a little east of south, from the Sierra Abajo aud 28 miles south- 

 west of El Late, and is a good example of the isolated mountain struct- 



