Bennettites dacotensis Macbride. — Calvin, 81 
or more, is tlius revealed: while beneath the Jurassic shahs. 
at the bottom of the basin-like valley, there is an exposure of 
Red Beds having a thickness of twenty or thirty feet. The 
rim of Big Horn basin, on tin- east side at least, exhibits ten 
or twelve feet of heavy, cross-bedded sandstone resting di- 
rectly on the Jurassic shales. These cross-bedded layers con- 
stitute the base of the great sandstone formation to which 
reference has already been made. The formation extends from 
the Jurassic shales to the summits of the adjacent buttes. On 
stratigraphical evidence we are now prepared to recognize it 
as the Dakota sandstone. The cycad beds are therefore Cre- 
taceous and belong to Meek and Haydens Cretaceous No. 1. 
A considerable thickness of the sandstone at the top of all 
the higher buttes of the region has been converted into a very 
hard, brittle quartzite. The process of vitrification has in 
some instances almost completely obliterated the original 
structure; in other cases the original sand grains are seen 
imbedded in a secondary deposit of silica. Contrary to the 
opinion of some observers. 1 believe the vitrification to be due 
to conditions that existed in the sea at the time the beds 
were deposited. The waters were charged with an unusual 
amount of soluble silica which was not only precipitated 
among the sand grains, converting the whole mass into 
a homogeneous quartzite, but some of it was substituted for 
the molecules of lignified and other tissues in the stems of 
cycads and deciduous trees that by accident were floated in 
from adjacent lands. 
The silicified trunks of ordinary trees now found on the 
lower slopes occupied by the sandstone are very much broken 
and weathered and polished by long exposure. On the shoul- 
der of one of the buttes, a mile or two west of the main cycad 
field, not far below the level of the vitrified bed, there was 
noted a silicified log two feet in diameter at the base, twelve 
feet of the basal part unbroken, and a train of fragments of 
varying dimensions extending from the smaller end far 
enough to indicate an original length of seventy or eighty 
feet. The fresh appearance of this specimen with it- frac- 
tures sharp-angled and its parts of considerable length all in 
their natural relative positions, was in striking contrasl with 
the short, polished, worn, disassociated fragments found in 
