EKDLicH.] BASALTOIDS DIKES. 113 



Peaks, Sierra La Sal, Sierra Abajo, La Plata Mountains, and others 

 are composed of it* In connection with porphyritic trachytes, dikes are 

 very often found that are more typically represented by uo other vol- 

 canic rock. 



Basalto'ids.- — In these I count dolerites and basalts, each with their 

 respective tuff's and conglomerates. Of the former but very little can 

 be found in the districts I examined, but the latter are well represented. 

 Almost "endless varieties present themselves, each one of which might 

 readily furnish an occasion for the creation of a new rock-species, were 

 they not unmistakably bound together when found in positti. Fre- 

 quently basalts are found covering the trachorheites. They unchang- 

 ingly preserve the same relative position — that of the younger groups. 

 As capping to plateaus and peaks they occur, occupying the most 

 elevated points. The most extensive basalt-series that I have noticed 

 in Colorado occurred on the eastern slope of the southern extension of 

 the Sawatch Eange. All along this slope, far into San Luis Valley, 

 the basalt extends in one uninterrupted flow. There, too, it covers 

 trachorheites. 



In addition to the occurrence as long-continued flows, which may, 

 however, at present be separated into isolated patches, are the local 

 small eruptions of basalt. Cone-shaped or rounded hills, sometimes 

 far from any mountains, protrude through the surrounding sedimentary 

 beds. Basalt, of several varieties, composes them. Again, it is found 

 as dike-material, second in importance to porphyritic trachyte. Most 

 frequently these occur in the younger sedimentary formations, rarely 

 older than the Cretaceous. Xo doubt they penetrate the older ones, 

 but the causes resulting in their formation did not exist until com- 

 paratively recent geological ages. 



Dikes. — Dikes are simply fissures in the " country- rock," injected with 

 some material that at one time must have been in a viscous or highly^ 

 plastic state. Generally, they are filled from below, but they may also 

 receive the material laterally. As a rule, the material filling the fissure 

 resists effectually the influence of atmospheric agents. Gradual dis- 

 integration of the adjoining rock causes the volcanic inclosure to stand 

 out prominently, representing essentially a cast for which the fissure 

 was the mould. 



Not infrequently the edges of strata forming the sides nearest the 

 dike may still be distinguished on the wall-like projection. Trachyte, 

 rhyolite, porphyritic trachyte, and basalt form the dikes of Colorado. 

 So far as can be judged, many of them were injected during a time 

 when the lava exhibited a very high degree of heat. This is apparent 

 from the metamorphosis which often the adjoining sedimentary beds 

 have been subjected to. Dependent upon the character of the meta- 

 morphosed rock this process may render it less liable to disintegration 

 and decomposition. Obviously the eventual result will be that the sides 

 of the dike remain hidden upon removal of the softer adjoining por- 

 tions, and that a hill, shaped like a hogback will be formed. Not all 

 rocks, however, are rendered more resisting by metamorphosis ; on the 

 contrary, many are led to a more speedy destruction thereby. 



We do not find only single, isolated dikes, but entire dike-systems. 

 Either radiating from one point, or running parallel with each other 

 or forming a net-work, or combining several of these forms, thej'^ are 

 capable of producing elaborate orographic results. Taking, for instance, 

 a case in which the passage of the highly fused lava has sufficiently 

 hardened the material through which it passed, we will readily perceive 

 that a radiating arrangement of such dikes must be productive event- 



