JURASSIC. 563 



shown that the same meta-andesite is younger than the Hardgrave sandstone. The general dip 

 of the Thompson hmestone is to the southwest beneath the meta-andesite by which it is bounded 

 in that direction, and both pass beneath the Hardgrave sandstone. All have been overturned 

 together. 



Hyatt's comment on tne age of the Thompson limestone, which he called the 

 Opis bed, is as follows : 



Mr. Ciller's close and repeated investigations of the stratigraphy have placed the Opis bed 

 below the Mormon sandstone in the chronologic series, and my studies, although they led me to 

 incline to the opinion that the fauna was younger, have not succeeded in bringing to light any 

 evidence that can be said to contradict his conclusions. The presence of a large form of Nerinea 

 with the columella, showing the typical ridges of the normal forms of this group, indicates that 

 this limestone is not older than the inferior Oolite, and if, as seems to be the case, it is older than 

 the Mormon sandstone, it will probably be proved to be a member of the inferior Oolite. 



A Jarge species of Opis is as abundant in some places as the Nerinea, and this genus, which is 

 recorded in Europe as beginning in the Trias, is usually small throughout the lower and middle 

 Jura. The only European species approximating to that of this limestone is the Opis paradoxa, 

 as figured by Buvignier, which occurs in the CoraUian of the upper Jura. A species of Tere- 

 bratula, apparently identical with the large characteristic species of the Mormon sandstone, also 

 occurs abundantly in this bed. There are also a number of small gastropods and other fossils 

 requiring further investigation. 



The Mormon sandstone consists prevailingly of sandstone, passing on the one hand into 

 conglomerate and on the other into more shaly beds. The most common and characteristic 

 member is a gray compact sandstone so fine that to the naked eye it does not appear granular. 

 Its color becomes brown on weathering and the rock frequently contains casts of a small Rhyn- 

 chonella. Among the grains of which it is composed there are many of quartz and of feldspar, 

 some of which is clear plagioclase with distinct twinning lamellse. A few of pale-green augite, 

 but most of them are of indefinite clouded material in which here and there small lath-shaped 

 crystals of feldspar may be seen, indicating their derivation from igneous rocks. There is a 

 small amount of carbonate of lime present, and it becomes somewhat more abundant in the 

 coarser forms. The conglomerate of the Mormon sandstone is generally greenish but some- 

 times reddish. Its pebbles are in a few places mainly quartzite, but at most locaUties they are 

 chiefly of meta-andesite with a few of metarhyolite, and range from one-eighth of an inch to 18 

 inches in diameter. Several pebbles of hmestone were observed, but none contained fossils. 

 In the conglomerate are masses of fine shaly red sandstone with decided pencil structure parallel 

 to its stratification. This same sort of material occurs also to a limited extent entirely indepen- 

 dent of the conglomerate, and in such cases it is thin bedded and finely stratified. * * * At a 

 narrow point in the wavy belt across the western slope of Mount Jura the following section was 

 observed: Adjoining the eastern side of the Thompson limestone is 45 feet of massive gray 

 sandstone with a rich fauna. This is succeeded farther up the slope by 30 feet of conglomerate 

 and finally by about 20 feet of finely stratified red shaly sandstone, the "Inoceramus bed" 

 mentioned by Prof. Hyatt. The total thickness of this belt a short distance farther south 

 increases to about 225 feet, and the conglomerate becomes more prominent but farther along 

 again diminishes and is scarcely noticeable near the south end. 



The maximum thickness of the Mormon sandstone noted by Diller is 550 feet. 

 Diller quotes Hyatt at length in regard to the fossils and age of the Mormon sand- 

 stone, and Hyatt concludes that the horizon is that of the upper part of the inferior 

 Oolite of Europe. 



The Bicknell sandstone is composed chiefly of red and gray sandstone associated with 

 some dark shales and tuflFaceous beds. The dark brownish-red sandstone is largely feldspathic 

 and contains much red oxide of iron, with more or less carbonate of hme. It somewhat resembles 



