238 GEOLOGY. 



Pliocene thins out towards the upper end of the valley, and near 

 the State line, in places where it overlies the Fort Pierre Cretaceous, 

 is only from twenty-five to sixty feet thick. The means at my dis- 

 posal did not enable me to determine the probable cause of this 

 phenomenon. 



The most silicious strata of the Pliocene in Nebraska contain 

 more or less of calcareous materials. The sandstones vary a great 

 deal in texture, the finest, as elsewhere, being generally beneath, 

 increasing in coarseness to the top, where, as already observed, the 

 rocks often assume the character of fine pudding stone conglomer- 

 ate. The pebbles, water-worn and smooth, are made up of all kinds 

 of rock, metamorphic materials being the most abundant. Among 

 these are granites, syenites, greenstones, quartz, sandstone and frag- 

 ments of silicified wood. This conglomerate, when decomposed 

 resembles the drift so closely that at first I mistook it for that for- 

 mation. Above the mouth of the Arickeree, and at other points in 

 southwestern Nebraska, the Pliocene is capped by an intensely 

 hard silicious stratum, from two to ten feet thick. It varies from 

 something akin to quartzite, to flint, hornstone and chalcedony. 

 The color of the latter varies from a creamy white to transparent, 

 and occasionally is coarsely opalized. A few moss agates are some- 

 times found in portions of this stratum. Prof. Mudge reports a 

 similar stratum in the Pliocene near Fort Wallace, Kansas. Ex- 

 cepting this hard layer, much of even the most compact strata is 

 apt to disintegrate on exposure to the elements. Sometimes the 

 most compact portion is so irregular in structure as to interfere with 

 the dressing of the stones for architectural purposes. 



Polishing Powder. — Infusorial Earth. — Geyser Plocula. — One of 

 the most remarkable of all the deposits of this Pliocene lake of the 

 plains, is a peculiar, flour-like material that appears in beds of 

 greater or less thickness and extent, that occurs on the Republican, 

 the Loup, Niobrara, and other sections. When I first examined it 

 under the microscope, eight years ago, a few diatoms were collected, 

 from which circumstance it was regarded as probably of the char- 

 acter of tripoli. Since then, in many specimens that have come un- 

 der my observation, a diatom has rarely been found. In almost 

 every specimen examined, however, great numbers of the forms that 

 Ehrenberg called Phytolitharia were detected. The most conspic- 

 uous of these are triangular in shape, with one edge convex and 

 the other concave, or the opposite. They cover, under a micro- 



