CHAPTER V 



By Whitman « !ros 



IGNEOUS FORMATIONS. 



SECTION L— GEOLOGICAL OCCURRENCE. 



INTRODUCTION. 



The igneous rocks occurring within or adjacent to the Denver Basin 

 occupy hut little space in comparison with the sedimentary rocks. In 

 making this statement the granites and gneisses of the supposed Archean 

 complex are not taken into account. These are no doubt igneous in origin 

 to a very great extent, but, as already explained by Mr. Emmons, no special 

 study of the pre-Cambrian rocks has been undertaken, and they are, 

 therefore, Left out of consideration. 



Of the rocks to be discussed, basalt is the only one of much importance. 

 From its occurrences conclusions may be drawn bearing upon several points 

 of structural or historical geology. 



Basalt appears on the plains, adjacent to the foothills, in dikes and in 

 surface flows or sheets. None of the latter were poured out upon surfaces 

 corresponding at all to surfaces of the present time, but they were originally 

 surface flows. All masses come undoubtedly from a common source, though 

 differing somewhat in composition and structure, owing to the varying 

 conditions attending their consolidation. The dikes are to be considered 

 as the channels through which the lava of the streams ami sheets came to 

 the surface. That they are now exposed at practically the same horizons as 

 the sheets which issued from them is due to dynamic movements which are 

 discussed in Chapter I. We now see but a remnant of the sheet which 

 once existed, and have little data for the estimation of its former extent. 



