242 REPORT UNITED STATES GEOLOGICAL SURVEY. 



points especially capable of resisting disintegration. Sbarp points on 

 ridges or even isolated bills in tbe adjoining valleys indicate tbeir po- 

 sition. 



Tbis system of mountain structure is one tbat bas not beretofore been 

 observ^ed, so far as I am aware. Until it was pointed out in tbe por- 

 pbyritic tracbyte groups it bad not appeared elsewbere. Perbaps tbe 

 best locality, one wbere it may be studied together witb its most suc- 

 cessful results, is tbat of tbe Spanisb Peaks. 



On a more extensive scale tban tbe preceding series is tbe second 

 one. Altbougb bere, too, tbe dikes enter very largely into considera- 

 tion, they are of but secondary importance. Kegarding tbe structure 

 scbematically, we may state tL at in tbis instance tbe volcanic material 

 is contained mainly at or near the surface. 



Having passed upward through fissures or tubes formed in compara- 

 tively hard strata, tbe lava finds such that will yield to lateral compres- 

 sion and to vertical separation caused thereby. This permits a horizontal 

 expansion of the lava. As compared witb the preceding group, this form 

 may be considered tbe more complete. In tbe one case all the volcanic 

 material is still confined within tbe limits of tbe original apertures of 

 amission, while in the other secondary openings are formed permitting 

 the entrance of foreign matter. 



In many instances it may remain a question in doubt whether any 

 but a very small portion of the lava originally reached the surface. 

 Primarily it would appear as if this question might readily be decided 

 by the shape of tbe mountain or mountain-group. Upon examination, 

 bowever, this involves many considerations. Wherever Jloivs have ex- 

 tended from the central regions of outflows, covering such strata as 

 w€re exposed at tbe time of eruption, there is no difficulty in making a 

 determination. Denudation may produce an effect easily mistaken for 

 surface-flows. Erosion, and disintegration subsequent to exposure will 

 produce results tbat may defy the recognition of tbe original condition 

 of the portions involved. From tbis it may be seen tbat very few ap- 

 plicable data can be obtained whereupon to base any opinion per- 

 tinent to the question. "While we have evidence that tbe sedimentary 

 beds have been arched to a very considerable elevation (Sierra La Sal) 

 and have sustained but few ruptures, it seems, a priori, improbable that 

 such should be tbe case in all instances. Gradual denudation can re- 

 move enormous quantities of material. It is not to be supposed, how- 

 ever, that it would remove a sufficient thickness of strata to permit the 

 assumption tbat most of tbe groups were formerly covered by sedimen- 

 tary deposits. The flexure of strata in the cases of the eruptions under 

 consideration is established. It seems probable, therefore, from the 

 very magnitude of the arches, that they broke at tbe points or along 

 tbe lines of weakest resistance, thus permitting an emission of large 

 quantities of the lava. As a rule, I think we may state that the cen- 

 tral portions of tbe groups reached tbe surface, but that much of the 

 lateral material now exposed was formerly bidden under a mass of sedi- 

 mentary rocks. Tbis will apply equally to nearly all of the groups ob- 

 served. 



IV.—RELATIONS TO SEDIMENTARY FORMATIONS. 



Groups of mountains composed of porpbyritic tracbyte are found 

 mainly in regions exhibiting Cretaceous beds. Among sucb we count 

 all the more westerly ones. A partial exception is made by West Span- 

 isb Peak. There the trachyte has mainly broken through, and is con- 

 tained in fissures formed in Carboniferous strata. At a few localities it 



