144 BULLETIN OF THE ESSEX INSTITUTE. 



orographic movements have affected this region many 

 times since the early Paleozoic epoch. It is certainly im- 

 probable that the Potsdam sandstone was, in any Post- 

 Cretaceous epoch, so imperfectly consolidated as the 

 formation of the minor sandstone dikes in the manner 

 here proposed, at so late a date, would require. I sug- 

 gest therefore that they probably date from some compara- 

 tively early movement along this line. In this connection 

 it may be noted that the numerous slickensides in the 

 dikes show movement after the lithifaction of the sand, 

 and therefore long subsequent to the first formation of 

 the dikes. 



It is a necessary corollary of the view developed here 

 that the sandstone dikes of Turkey Creek, Nipple Moun- 

 tain, and possibly other localities in this region, indicate 

 formerly overlying Potsdam strata, and thus throw light 

 upon the former distribution of that formation. They are 

 narrow linear Grdhen formed along sheeted zones, or more 

 literally trenches in which portions of the Potsdam sedi- 

 ments have been buried below the present plane of erosion, 

 and thus preserved. As erosion cuts more deeply, all the 

 sandstone dikes will disappear, as they practically have 

 done already between Cascade and Green Mountain Falls, 

 or where the Potsdam beds are wantino-. 



The sheeting of the granite which this theory demands 

 is by no means a purely theoretical feature. On the con- 

 trary, this type of jointing may be regarded as more or 

 less characteristic of the granite of Ute Pass, as may be 

 so well seen at Rainbow Fall. And two striking illustra- 

 tions of sheeted zones in a distant part of the Pike's Peak 

 Massif accompany the recent monograph by Penrose on 

 the Mining Geology of Cripple Creek. ^ 



•^^ lotli Ann. Rep. U. S. Geol. Survey, part 2, plates ill and iv. 



