328 GEOLOGICAL SKETCHES OF 



ern section, form the basis of a peculiar, metalliferous, 

 and in some parts, elevated range called the Ozark 

 mountains. 



West of these mountains, in about longitude 19° 

 W. is the commencement of the Great Desert, a re- 

 gion of granitic sands, consisting of the detritus of the 

 Rocky mountains, and exteuding westward to the base 

 of that range. 



In longitude 28° W. a narrow crest of argilla- 

 ceous and ferruginous sandstone, the latter of an in- 

 tense red colour, emerges from beneath the deep and 

 loose sands of the Platte, reposing in a highly in- 

 clined position against the granite of the Rocky moun- 

 tains. 



In this latitude, the Rocky mountains, on their east- 

 ern side, are almost exclusively of granite, in which 

 a reddish yellow feldspar is predominant and hi rn- 

 blend supplies the place of mica. About the lower 

 parts of the mountains, mica occurs in small quantity, 

 and is usually of a very dark colour. 



SOUTHERN SECTION. 



The eastern declivity of this range of mountains 

 in latitude 33° N. the parallel of :he southern section 

 is covered by extensive formation of porphyrliic 

 and amygdaloidal greenstone and other rocks c Hed 

 Floetz trap by some geologists. Here, as in Eng- 

 land, Germany and many parts of Europe, these roi ks 

 are in immediate association with the < «> >1 strata, on 

 which they are sometimes superimposed in iuiu*euae 



