JURASSIC. 565 



Almost as abundant as the shales are the sandstones, most of which are very fine and 

 decidedly shaly. They are reddish brown and gray, sometimes mottled with darker spots, 

 chiefly carbonate of lime. * * * Occasionally the gray sandstone is more sihceous and 

 contains irregular particles which give to the mass the appearance of a fine tuflaceous con- 

 glomerate. 



Conglomerate is less abundant than either shale or sandstone and is usually in thin beds 

 of small extent scattered throughout the formation. Tha most characteristic conglomerate 

 of this formation is composed almost wholly of small pebbles of cherty quartz, black and gray 

 in color, and rarely over half an inch in diameter. Of this there are but f sw beds ranging from 

 5 to 12 feet in thickness. A more abundant form of conglomerate contains locally many shale 

 fragments and well-rounded pebbles chiefly of meta-andesites and rhyolites, with a few of 

 fossihferous limestone. The majority of the beds are small and are made up of small pebbles, 

 rarely as large as 3^ inches in diameter. Associated Avith the limestone pebbles in the con- 

 glomerate are concretionary nodules of carbonate of lime, which occur also in the shale and 

 sandstone. They do not contain fossils and grade into the inclosing rock instead of having a 

 sharp contact, as is the case with the hmestone pebbles. 



The Foreman formation contains fossil plants, which were studied by Fontaine, 

 who placed them in the Mesozoic and considered them as most probably older than 

 Cretaceous. An examination of later collections by Knowlton led him to question 

 some of Fontaine's determinations, and marine invertebrates in the formation 

 suffice to prove that it is Middle Jurassic or later. Stanton is of the opinion that 

 it may represent the youngest Jurassic of the region, and this view is confirmed 

 by Diller's study of the stratigraphy. 



The Myrtle formation of southwestern Oregon and the Knoxville of northern 

 California are regarded by DiUer, Stanton, and Knowlton as of the same age, but 

 it is a mooted question whether they are wholly Cretaceous or in part Jurassic. 

 According to Stanton, the Aucellas (Aucella piochi and A. crassicolis) show that 

 the formations are not older than Lower Cretaceous. According to Kriowlton, 

 the plants which are associated with the Aucellas at some localities are characteris- 

 tically Jurassic." The Myrtle and Knoxville are indicated on the map as Lower 

 Cretaceous, largely to distinguish them from the adjacent Franciscan areas, which 

 are colored as Jurassic, and without any intention of indicating a decision as to 

 their Cretaceous or Jurassic age. 



K 12. SOUTHWESTERN WYOMING. 



In the survey of Uinta County, in southwestern Wyoming, Veatch^^"" distin- 

 guished the Twin Creek formation and the lower part of the Beckwith formation as 

 Jurassic. The Lower Cretaceous appears to be missing or to be represented in the 

 upper part of the Beckwith, as the Bear River formation (Upper Cretaceous?) 

 immediately succeeds the Beckwith, apparently without unconformity. Veatch 

 gives a general description, which is quoted in the table on page 676, Chapter XV. 

 From his report we take the following notes : 



The fossiliferous marine Jurassic, to which has been given the local name Twin Creek 

 formation, from the excellent exposures on that creek between Sage and Fossil, here consists 

 for the most part of dark calcareous shales and thin-bedded shaly hmestones, though occa- 

 sionally sho-\ying lighter-colored sandstone layers. These are sharply hmited above by the 

 thick red beds which mark the base of the Beckwith formation. The thickness of the Twin 



« 9ee quotations under K 10 in Chapter XIV, pp. 617-621. 



