162 



GEOLOGICAL SUKVEY OF THE TERRITORIES. 



^ig- 41- The entire exposure of 



limestones is about 800 feet 

 in tliickness. The upper, 

 I or rather lower, layers — 

 "5 for they are inverted as I 

 found them in th« middle 

 ~ canon— are nuignesian, and 

 . contain in jdaces bladed 

 crystals of tremolite. The 

 limestones are followed by 

 430 feet of massive quart- 

 zites, which are at first 

 \f,f' r-ni' /i 't-T- "^^'^'^^^^ '^-"^^ tlien dark from 

 '^'if J^ vv^'^^^^'— ^^^ presence of iron. Be- 

 %^^^01 nPfith the quartzites, and 

 "'^-^'-'^ dipping in the same direc- 

 ^ tion at an angle of 20°, are 

 I alternate layers of sandy 

 ^ and calcareous shales, with 

 interlaminated bands of 

 coal-like slates, which 

 break readily at right an- 

 gles to the plane of deposi- 

 tion. They are followed 

 by greenish-gray calcare- 

 ous sandstones, breaking 

 into lamina from J inch to 

 2 or 3 inches in thiclaiess. 

 These contain Trigonia, Mo- 

 diola, and other fossils, 

 proving their undoubted 

 Jurassic age. All of these 

 rocks are conformable to 

 each other. 



Jackass Cjeek joins the 

 Madison at the lower end 

 of the valley, just above 

 the lower canon. This 

 creek, like the others, cuts 

 deeply into the range, 

 afibrding an excellent 

 chance to get at its struc- 

 ture. The range, however, 

 is so long and so rugged, 

 capped with so many sharp 

 peaks, that to determine 

 its geology with precision 

 will require the work of sev- 

 eral seasons, and I will be 

 able, therefore, to give only 

 a general idea of it. Still 

 I think future research 

 will modify but little my 

 idea and only fill in the de- 

 tails that are wanting. 

 There is no doubt but that 

 it presents one of the most 



INVERTED BEDS OF JACKASS CREEK. 



