GEOLOGICAL SURVEY OF THE TERRITORIES. 163 



interesting fields of study to be found in the West. Jackass Creek 

 emerges from the mountains tlirough a canon, the mouth of which is 

 very narrow and bordered by gneisses, which are very micaceous and 

 of a black color. As we proceed up the stream they become lighter. 

 The exposure on the right bank is very fine, the bedding being con- 

 spicuous, di[)ping northwest at an angle of 2(P to 30°. About a mile 

 inside the caiion we meet first with the limestones, which seem to dip 

 underneath the gneissic rocks. On the left-hand side of the canon, 

 as we go up stream, there are two hills, on one of which the outcrop 

 is gneissic and on the other limestone. Between is a deep gully, in 

 which the line of junction is so obscured that even with the most 

 careful investigation it could not be determined. Just above the 

 limestones we find shales and quartzites, which probably represent the 

 Potsdam group. Between there is a very compact porphyritic rock, 

 which I take to be an old intrusion of igneous material. The limestones 

 are very compact and brittle, and in layers of about (3 inches in thick- 

 ness, dipping northwest at an angle of 30°, seemingly conformable with 

 the gneisses. They are probably Silurian, and we found in them the 

 fragment of a tribolite and lingulepis. Still farther up the caiion we 

 find Carboniferous limestones, containing well-defined Carboniferous fos- 

 sils, dipping northwest, conformably, with the Siluriaubeds. These layers 

 above weather a blue color, those below yellow, and between them red. 

 Then follows a porphyritic layer, resembling the one seen above the 

 Silurian rocks. It is followetl by what I take to be Jurassic beds. 

 Although I found no fossils here, the rocks are precisely of the same 

 character as those I found in Bear Creek, which contained Trigonia^ 

 &c. Then followed more modern beds, made up of soft, gray cal- 

 careous sandstones and clay-slates. The lowest layer observed was 

 igneo^is material. The accompanying illustration, Fig. 41, corresponds 

 with the following section : 



1. Igneous rock. 



2. Clay-slates. 



3. Soft, gray sandstone. Containing fragments of leaves. 



4. Green and black shales. 



5. Gray shales. 



6. Green and black shales. 



7. Quartzite. 



8. Shales. 



9. Quartzite. 



10. Limestone. 



11. Conglomerate. 



12. Porphyritic, igneous rock. 



13. Dark-blue limestone. 



14. Reddish limestone. ^ 



15. Yellowish limestone. > Spirifer, ProductuSj and Zaphreretis. 

 IG. Bluish limestone. ) 



17. Silurian limestones. — Trilobite and Lingulepis. 



18. Shaly sandstones. 



19. Porphyritic, igneous rock. 



20. Quartzite. 



21. Gneiss. 



Toward the north the strike of these rocks seems to bend more and 

 more toward the east, and probably extend across to the Gallatin, where 

 I shall refer to them again further on. 



The valley of the Madison from the middle caiion to the lower canon 

 is fifty miles long and averages six miles in width. It once formed 



