tKDLicn.] BASIC VOLCANIC EEUPTIVES MASSIVE BASALT. 249 



tion. Evidently large masses of tbe lava were poured out at ouce, to 

 be followed by similar eruptions at later periods. Tiiose ejections that 

 have built up the mountains were very limited in quantity as com- 

 pared to the others, and owe to this fact, probably, their inability to 

 cover any considerable area. The entire region is one of great interest^ 

 showing, as it does, the relations between previous and basaltic volcanic 

 periods, besides illustrating the magnitude of the scale upon which such 

 erui>tions took place. 



ZFncompahgre Mountains. — Basalt occurs in the Uncompahgre Mount- 

 ains at considerable elevations. It was found on several high plateaus. 

 Upon one of them several stations were located,* and from there former 

 connections could be traced that indicate a rather extensive basaltic 

 area. The basalt is poured out over the older trachorheites, and occu- 

 pies a perfectly conformable position. An absolute elevation of over 

 12,000 feet can be observed on the summit of such plateaus. Another 

 one is found near the junction of Lost Trail Creek and the Eio Grande. 

 Several of the high peaks show caps of basalt. From the detached 

 fragments that we now observe, and their relative position, we can infer 

 that at one time the larger portion of all of them were at one time con- 

 nected. So far as observed the lithological character of the basalts 

 there is perfectly uniform, which argues for former connection. 



In speaking of rhyolite a locality was mentioned not far from Un- 

 compahgre Peaks where basalt was found under the former. This I re- 

 gard as one of the points of eruption, if not the only one. It is the only 

 place where basalt is found at a low elevation. As a rule the thickness 

 for the basalt of this region may be stated as 600 to 800 feet. 



Grand Mesa. — Northwest of the localities just described we find an- 

 other extensive basaltic area.t South of the junction of the Gunnison 

 and the Grand a high prominent mesa is formed by Tertiary beds. A 

 layer of basalt caps it throughout its entire extent, attbrdiug protection 

 to the easily-eroded underlying strata. Dr. Peale has described this 

 region. He gives the thickness of the' basalt as about 250 feet, but 

 expresses the opinion that this does not represent its original vertical 

 extent. It is probable that erosion has removed a large portion of the 

 basalts, and that we now have before us but the remnants of areas that 

 at one time must have been very extensive. 



On Eagle River and a portion of its drainage basaltic areas occur 

 that at one time were probably connected. In South Park, and associ- 

 ated tvith the trachorheites of the Front Eange, we find basalt expos- 

 ures that must be reierred to massive eruptions. 



So far as observed, all of these bear the same relations to either sedi- 

 mentary or older volcanic rocks. With these latter they are generally 

 conformable when found in contact. Although so distinctly offset in 

 almost every respect from other eruptives, the basaltoid group undoubt- 

 edly has been governed by the same laws that have determined the 

 j)0int8 or lines of eruption for other volcanics. They occur in the same 

 manner, though evidently of later date, they show the same methods of 

 protrusion, and they are analogous, in the results produced, to the less 

 prominent members of preceding groups. From their scheme of hori- 

 zontal and vertical distribution, it may be inferred that the basaltic 

 lava reached the surface in a very high state of viscidity. This cannot 

 always be said of the older volcanics. Taking advantage of the con- 

 figuration of the country at the time of ejection, the basalt has found 

 its way in the largest quantities to the points most readily accessible. 



* Eep. U. S. Geol. Surv., 1874, p. 202. t Kep. U. S. Geol. Surv., 1874, p. 174. 



