162 



GEOLOGICAL SURVEY OF THE TERRITORIES. 



Fig. 41. 





INVERTED BEDS OF JACKASS CREEK. 



The entire exposure of 

 limestones is about 800 feet 

 in thickness. The upper, 

 or rather lower, layers — 

 for they are inverted as I 

 found them in the middle 

 canon— are magnesian, and 

 contain in places bladed 

 crystals of tremolite. The 

 limestones are followed by 

 430 feet of massive quart- 

 zites, which are at first 

 white and then dark from 

 the presence of iron. Be- 

 neath the quartzites, and 

 dipping in the same direc- 

 tion at an angle of 20°, are 

 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 ^ inch to 

 2 or 3 inches in thickness. 

 These contain Trigonia, Mo- 

 diola, and other fossils, 

 proving their undoubted 

 Jurassic age. All of these 

 rocks are conformable to 

 each other. 



Jackass Creek 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, 

 affording 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 



