AREAL GEOLOGY OF LOWER CLEAR CREEK 



267 



only certainly pre-Jura-trias, and then only for the main masses. That 

 certain portions are of late eruptive origin can be proved, and possibly all 

 of the country will some day be shown, as in the case of the Green Moun- 

 tain area in Massachusetts, to be as late as Devonian. 



That this supposedly pre- Cambrian complex was originally igneous 

 and since metamorphosed by great dynamic forces, regional metamor- 

 phism, and thus as a consequence has taken on a well-defined schistosity, 



^ 



LEGEND 



FUNDAMEINTTAL ClEAR CREEK 



Gneiss Gneiss 



DiORITE 



Latite 

 Porphyry 



Sediments 



Fig. 4. — Portion of the Black Hawk Quadrangle. 



is borne out by its comparative homogeneity over large areas in the field, 

 and by the thin sections studied. It is difficult to beheve that alternating 

 strata and formation of sedimentary rocks could have been metamor- 

 phosed into such areas of homogeneous material as at present exist, and 

 the appearance of the lithologic units as mapped up to the present 

 strongly give the idea of an arrangement by igneous forces. While the 

 complex of the lower Clear Creek area may be divided into a number of 

 lithologic units with more or less certainty, in all probably about eleven 

 or more, only ten can be recognized with certainty in the area in question. 

 These formations beginning with the oldest, the Fundamental Gneiss, 

 have been arranged as follows : 



