ENDLICH.J ACIDIC VOLCANIC ERUPTIVES — POEPH. TB.ACHYTE. 239 



SIEKRA LA SAL. 



The Sierra La Sal, or Salt Mountains, is properly composed of three 

 groups, with, as Dr. Peale says*, "sedimentary saddles separating three 

 eruptive centres." During 1875 he visited that region and examined 

 carefully the structure of the raountains.t Rising to a relative elevation 

 of 8,000 to 8,500 feet above the adjoining river-valleys, these mountains 

 are located just west of the western boundary of Colorado, at about 

 north latitude 38° 30'. 



The entire structure of the mountains is a simple one, being mainly 

 a vertical eruption along a line trending approximately north and south. 

 Newberry in 1859 already noticed the probable eruptive character of the 

 group and compared it to that of the Sierra Abajo.f From Dr. Peale's 

 sections, it appears that the La Sal Mountains owe their formation to the 

 same process that caused the elevation of the La Platas and other 

 closiely-related groups. Instead of having but one fissure for the ejec- 

 tion of the main mass of volcanic material, we find two, running paral- 

 lel, in the middle and northern groups. This is accompanied by a A^aria- 

 tion from the simple, dome-shaped flexure of the sedimentary strata. 

 Most strikingly is this demonstrated in the middle group. There we 

 find two mountains, both of them of nearly 13,000 feet altitude. While 

 the Lower Cretaceous strata dip away both to eastward and westward of 

 the entire group, they have been raised to a considerable elevation be- 

 tween these two mountains. At present they form a saddle between 

 them, and are curved in the ibrm of a synclinal fold. Evidently the 

 force exerted in either mass of volcanics was not sufflciently great to 

 carry it high enough to obliterate this remnant of sedimentary strata. 

 Triassic and Jurassic beds have heen severely affected by the trachytes 

 and their eruption. Metamorphosed fragments testify to the heat, 

 while broken and disrupted strata indicate the enormous force employed. 



Interleaving of volcanic material with sedimentary beds may be ob- 

 served nearthejunctionsof the main fissures with the edgesof strata torn 

 apart. Apparently the Palaeozoic groups take part in the general flexure 

 of strata, indicating the deep-seated source whence the trachytes were 

 derived. 



The rocks, so far as observed, agree closely with those from other 

 localities belonging to the same class. Unfortunately the specimens 

 were lost, but sufBcieutly detailed examinations had been made in the 

 field to establish their character. 



SIERRA ABAJO. 



The Sierra Abajo was visited by Mr. Holmes during 1876, and his re- 

 port thereon is published in this volume, pp. 189, 193. Dr. Peale gives a 

 short synopsis of its character,§ which shows it to be perfectly conforma- 

 ble in its features with the other groups composed of porphyritic trachyte. 

 The range, if it may be so called, is situated due west of the San Miguel 

 Mountains, at about north latitude 37° 50', west longitude 109° 30'. A 

 number of prominent points compose the group. Evidently, as in other 

 regions of similar construction, the lava has been ejected through one or 

 more fissures, and, upon reaching the yielding Cretaceous shales, has 

 spread laterally. Thus intrusive masses are formed, similar to those 



*Bull. U. S. Geol. Surv., 1877, No. 3, vol. iii, p. 558. 

 t Rpp. U. S. Geol. Saw., 1875, p. 59. 

 t Expl. Exp., by J. N. Macomb, 1859, 1876, p. 93. 

 $ Bull. U. S. Geol. Sarv., vol. iii, No. 3, p. 558. 



