GEOLOGIC STJirCTUKE. 



393 



6 to 10 feet wide and of varying height, wliich cross the country in great 

 numhers, and are clearly seen to converge toward a common center. As 

 many as 70 distinct and separate examples were observed in one view. 



These relations are })resented on the accompanying geological map (see 

 figure 1), which shows the areal disi)osition of the varied rock masses 

 composing the mountains. 



FlGlRK 



■Geolofjicnl Map of the Ilirjhwuod Mountains Montana. 





Acid tutfs 

 and Hows. 





Q 





■ 



m:A 



nz 



Massive gran- 

 ular rocks. 



H:isi0 luffs 



and flows. 



Intruded dikes 

 and sheets. 



Cretaeeous 

 sediments- 



THE SEDIMENTARY PLATFORM. 



Although thr chief interest in the Jlighwood mountains lies in its 

 igneous rocks and their relations, a few words on the sedimentary heds 

 will be necessary. 



The sedimentary platform on which the mountains rest is formed of 

 soft Cretaceous strata. The general exi»ression of the jilatform is that 

 of a continuous level i)lain,l)Ut the topographic details possess consider- 

 able variety. The radiating streams from the mountain amphitheaters 

 have trenched the soft rocks, producing canyons which separate long 

 tables of nearly level prairie. So abrui)t are these gorges that in ap- 

 proaching even the largest of them, the valley of Belt creek, no indication 



