234 EEPORT UNITED STATES GEOLOGICAL SURVEY. 



positiou of the dike-walls it is apparent that an enormous amount of 

 sedimentary material must have been removed since the eruption took 

 place. Furcation and crossing of the dikes is also noticeable in this 

 locality. Wherever an additional strain was exerted on the strata they 

 broke in some other direction, thus causing the effects now observed. 



As might be expected, the passage of tiot material in a viscous or 

 plastic state has seriously aflected the sedimentary rocks with which it 

 came in contact. It has been stated above that the lava passed through 

 Carboniferous strata. These are composed of sandstone, with thin beds 

 of shale. In examining the contact between these rocks, it will be found 

 that the sandstones are thoroughly baked, altered into quartzites, and, 

 in rarer instances, into an aggregate closely resembling granite. Shales 

 are metamorphosed into hard, brittle argillites. It may be observed 

 that a short distance off from the dikes or intruded masses the meta- 

 morphosis becomes less pronounced, and the normal constitution of the 

 rock again appears. So far as could be seen, no complete fusion of 

 inclosing and inclosed rocks took place. This demonstrates that either 

 the degree of heat which the lava showed was not a very high one, or 

 that it lasted but a short time. 



It is a very difficult matter to arrive at aoy conclusions regarding the 

 degree of heat which lava contained at the time of its being injected 

 into fissures. So much we can say in the present instance, that the ma- 

 terial must at least have been in a plastic condition. Not only does the 

 complete filling of fissures and interstratal openings point to such a con- 

 clusion, but more direct evidence is also not wanting. On the sides of 

 a number of dikes we find the impression produced by the edges of strata 

 formerly in contact therewith. In other words, the fissure or '' gash " 

 in the sedimentary rocks may be termed the mould, while the dike itself 

 is the cast thereof. Frequently such marks are so well preserved that 

 it can be determined whether the rock inclosing the volcanic material 

 was, for instance, sandstone or shale. 



Some of the dikes evidently flowed over upon the surface at the time 

 of eruption, forming small, regular hills or buttes. More frequently can 

 this be observed in connection with those filling the largest fissures. 



Upon the sedimentary beds and their absolute positiou this eruption 

 has had a very definite effect. It has raised the entire mass, a priori^ 

 has produced vertical distention by entering interstratal fissures, and 

 has resulted in horizontal expansion. Had not the fissures been tilled, 

 many or most of them would have closed agaiu. As the separated edges 

 of strata were kept apart, however, by tbe lava, which soon assumed 

 rigidity, the primary lateral and vertical displacement was retained. 



It is apparent that so singular a mountain-structure must manifest 

 Itself in a marked manner on the exterior. Viewing Spanish Peaks from 

 a short distance, the first striking feature noticed is the regularity and 

 sharpness of their ridges. Added to this, we observe the symmetry of 

 the entire structure and the striking singularity of minor details. Ex-' 

 amination develops the fact that nearly every one of the ridges leading 

 to the summit is surmounted by a dike. This dike, together with the 

 hardening of the contiguous strata it has caused, has resulted in the 

 formation of the ridge itself. Due to the regularity of the radial ar- 

 rangement of the dikes, therefore, is the symmetrical distribution of the 

 ridges. Between the ridges the sedimentary material has not been 

 reached by the power of metamorphosing agents, hence has readilj^ suc- 

 cumbed to eroding influence. As the result, we find deep gorges, grow- 

 ing very narrow toward the bottom, separating the individual ridges. 



Among all the mountains that have come under my observation, none 



