LITTLE SACRAMENTO GULCH. 151 



of inconsiderable magnitude, or of quite small intrusive masses which doubt- 

 less are the upper portion of similar dikes whose base is concealed. It is 

 probable that these minor eruptions of porphyry are of later date than the 

 main intrusive masses which prevail to the southwest of this imaginary line. 

 Although the Sacramento Porphyry is not found upon the surface to the 

 west of the main crest of the range, it is probable that it did not originally 

 end abruptly there, but gradually thinned out in some such form as is indi- 

 cated in Section D, west of the Mosquito fault, or as is shown more in detail 

 in the sections accompanying the Leadville map. Lithologically it forms a 

 definite type, whose general character has already been given in the chap- 

 ter on Rock formations. Its distinguishing characteristics, as compared with 

 the other porphyries, are its relatively large proportion of plagioclase feld- 

 spar and its carrying hornblende. These ally it in some degree to the por- 

 pliyrites. 



Little Sacramento gulch. The observations made in Little Sacramento 

 gulch, which time did not admit of repeating, were unfortunately not suffi- 

 ciently detailed to afford data for an accurate outlining of all the bodies of 

 porphyry found there. The principal uncertainties resulting herefrom are: 

 first, as to the eastern limit in the gulch of the main body of the Sacra- 

 mento Porphyry : whether it confines itself to the horizon which it follows 

 with apparent regularity farther north or whether it cuts across the over- 

 lying beds ; and, secondly, whether a body of the same porphyry observed 

 on the north face of the ridge separating Little Sacramento from Spring 

 Valley is connected with the main body as a transverse body, or whether 

 it is a portion of an interbedded sheet, like those on the western face of Lon- 

 don Hill, with which it might be possibly connected by the bodies observed, 

 but not outlined, on the eastern flanks of the Gemini Peaks ridge. 



Kast of the fault, it is evident that in the region included between 

 Horseshoe and Big Sacramento gulches there is a lateral syncline similar 

 to that observed on Pennsylvania and Lovelandllills, but broader and deeper. 

 The sin-lace of the region is too much covered to admit of this fact being 

 determined by the observed dip of the beds, but it is evident by the fact 

 that the erosion of Little Sacramento gulch, where it lraver<e> the arch of 

 tli- Sheep .Mountain fold, has cut down either to a very little depth or not 



