JUKASSIC. 561 



These facts and the evident variation in thickness of the massive Dolores sandstone, which 

 we noticed at many places, seem to speak for a relation of the La Plata and Dolores very similar 

 to that existing on the western and southwestern slopes of the San Juan Mountains. But much 

 more careful observation is needed in the Uncompahgre Plateau to determine to what extent 

 the absence or variable thickness of the Triassic beds is due to pre-La Plata erosion. Person- 

 ally, I believe that the Triassic beds were originally deposited over the Uncompahgre Plateau 

 and indeed all of western Colorado, and that they were in some places entirely removed by the 

 erosion under discussion, but this view is not yet supported by enough evidence to warrant a 

 positive assertion. 



K 10. UPPER SACBAMENTO VALLEY, NORTHERN CALIFORNIA, AND SOUTHERN OREGON. 



In northern California several minor but interesting occurrences of the Jurassic 

 have been described by Diller.^''* The Modin and Potem formations occur in the 

 Redding quadrangle. The Modin consists of a basal conglomerate, followed by a 

 succession of tuffaceous beds, which are overlain by gray shaly sandstones and shales 

 with small limestone lentils. It rests unconformably upon the Triassic, and the 

 tuffaceous beds include fragments of the underlying limestone and other older forma- 

 tions. The Potem formation consists of shale and thin-bedded sandstone with small 

 limestone lentils^ and it also includes some tuffaceous conglomerate. The thick- 

 nesses are stated by Diller at 3,000 feet for the Modin formation and probably several 

 thousand feet for the Potem. 



In the Taylorsville district, just north of the fortieth parallel, the Jurassic is 

 represented by seven distinct formations. According to Diller ^*°^ — 



The Trail formation is the oldest; then follow the Hardgrave sandstone, Thompson lime- 

 stone, Mormon sandstone, Bicknell sandstone, Hinchman tuff, and Foreman formation in regular 

 succession. With the exception of the last their surface distribution is limited to a compara- 

 tively small area about Mount Jura, and * * * Jt j^ay i^e stated here that the whole set of 

 beds have been overturned and that the Hardgrave sandstone apparently lies on top. * * * 



The Trail formation includes a mass of strata composed largely of slaty shales with some 

 interbedded sandstones and conglomerates. * * * ft^g shales are often purplish or red but 

 perhaps more frequently gray, with pencil structure locally developed, and contain in places 

 numerous cherty nodules of carbonate of lime. Well-marked slaty structure is not uncommon. 

 The sandstones are generally fine, often somewhat slaty, are thin-bedded, and vary from gray to 

 almost black. The conglomerates of slate and sandstone pebbles, sometimes 3 inches in diam- 

 eter, generally contain much volcanic material. Approaching the contact with the granodiorite 

 in many places the fine sediments become darker and more compact, with conchoidal fracture, 

 and pass into regular hornfels. * * * Some of the beds of tuff are well defined, ranging from 

 10 to 50 feet or more in thickness, and occasionally coarse. Tuffaceous conglomerates are most 

 abundant on the side toward the Swearinger slate but extend throughout the formation, and a 

 strip of 50 feet of slaty conglomerate occurs in the volcanics which bound the sediments on the 

 northeast. * * * Tj^e thickness of the Trail formation was measured on the prominent spur 

 next north of Hornfels Point descendiag into the sharp bend of Hosselkus Creek and found to be 

 about 2,900 feet. 



Undeterminable traces of plants and some smaU bivalves, possibly Estheria, 

 but not any fossils which would determine its horizon, have been found in the Trail 

 formation. It appears to be unconformable to the Swearinger slate, and thus to 

 occupy essentially the same relative position as the Modin formation^ at the base of 

 the Jurassic sediments. 



48011°— 12 36 



