496 



PECULIARITIES OF THE GRAVEL NEAR BATH. 



done on the southeastern or outward slope of this southeast rim. And I think it not at all im- 

 probable that this outer slope (which in the New Jersey mine is 900 feet long) may once have 

 formed a portion of another great channel, parallel, or nearly so, with that which now seems to 

 exist farther back in the hill. In that case, however, this front channel must have extended far 

 out to the southeast over the present canon of the Middle Fork of the American, and a great por- 

 tion of its breadth must have been carried away in the excavation of that canon. But this is 



speculation. 



In order to appreciate the full force of the evidence furnished by the boulders of white lava in 

 the banks at Bath, of their having come from the Long Canon country, and the adjacent region in 

 the upper basin of the Middle Fork of the American, the attention should be more particularly 

 directed to one point which I omitted to mention, that is, to the texture of the rock which forms 

 these boulders. A few words here with reference to the character of the white lava in general 

 will set this point in clearer light. The statement previously made* must not be understood as mean- 

 ing that I saw nothing in the country north of the Middle Fork of the Middle Fork belonging to 

 the same great class of material as the white lava, but only that I saw nothing at all nearly 

 resembling that particular variety of it which forms so many of the boulders at Bath, and which 

 is common in the country farther south. The gray, or grayish white, granitic-looking sand 

 cement, which forms the roofs of the mines so extensively at Bath and elsewhere in the Forest 

 Hill ridge, belongs undoubtedly, I think, to the same general class of material as the white lava 

 farther south. The same may be said of the material of an outcrop on the road near Independence 

 Hill, in the Iowa Hill ridge, of which, I believe, I took specimens. 



Moreover, the roof of the tunnel I entered at the Strawberry mine near Monona Flat, though a 

 very dark-colored substance, and pretty hard, is probably a similar material. I take it all to be 

 volcanic ash. But wherever seen to the north of the Middle Fork of the Middle Fork of the 

 American (and it was not seen in any great quantity in any part of this region, excepting from 

 Bath southwesterly in the Forest Hill ridge), it was always comparatively coarse-grained, rather 

 soft, of rather loose texture, and sharp, harsh, gritty feel, and generally contained a notable quan- 

 tity of black mica in scales, often the twentieth, and sometimes the tenth of an inch in diameter, 

 giving the whiter varieties of it that peculiar pepper-and-salt appearance so strikingly suggestive 



of granitic sand. 



But the white lava varies from this coarser-grained and loose material all the 



way down through finer and finer grained varieties to a rock of perfectly compact and almost 

 flinty or semi-opaline appearance, with small transparent crystals — -probably of glassy feldspar — 



scattered through it here and there. This most compact variety is not always white. 



It some- 



times assumes a brownish, and not unfrequently a pinkish tinge. And it is precisely this compact 

 variety which forms many of the boulders at Bath, and of which there are large quantities in 

 place in the southwestern end of the ridge between Long Canon and the Middle Fork of the Mid- 

 dle Fork, at a rectilinear distance of only some six or seven miles from Bath, and of which I did 

 not see a particle elsewhere north of the Middle Fork of the Middle Fork. 



It is curious that to the north of this stream the white volcanic ashes, so far as seen, should be 

 almost universally coarser-grained, — this most compact variety being nowhere seen, — while in 

 the country south of it, where also the mass of the white lava occurring is vastly greater, the 

 compact variety is not uncommon, and the rather finer-grained varieties with little mica seem the 

 rule, and the coarser-grained micaceous ones are far less common. 



The proportion of mica in the white lava varies as greatly as it does in volcanic rocks in 

 general. Some varieties are almost absolutely free from it, while in some the proportion is very 

 large ; and there is every grade between. 



It is worthy of note that the white lava, wherever it occurs, so far as I have seen (and in many 



■ 



places its quantity is simply enormous), is always segregated by itself, that is, it never contains 

 large boulders or much foreign matter of any kind. I do not remember having ever seen im- 

 bedded in it a stone so large as the two fists. Yet it almost everywhere contains occasional 



* See ante, p. 494. 







