564 INDEX TO THE STRATIGEAPHY OF NORTH AMERICA. 



the red Hardgrave sandstone but is darker colored, and though much of its material may be 

 derived from igneous rock, its particles are not characterized by the presence of numerous 

 small crystals of feldspar as in the Hardgrave sandstone. Sandstones, mottled gray and red, 

 form transitions to the gray sandstone which is the most abundant rock of the formation. 

 Much of it is fine dark bluish gray, very compact and hard, breaking with a conchoidal fracture, 

 and flecked here and there by small particles of pyrite. It looks so much Hke homfels that 

 one is surprised to find it locally full of large shells. Associated with tliis form is more or less 

 black shale, some beds of which weather white and look Hke fine volcanic dust. A thin section, 

 however, shows no characteristic volcanic material, but instead extremely fine sediment with a 

 multitude of minute microscopic crystals of rutile which have developed in the sediment since 

 it was laid down. 



A coarser variety of the sandstone is light gray or greenish and passes into a rock which is 

 full of small white crystals of feldspar with a few scales of gray mica embedded in a dark ground- 

 mass, and the general aspect is decidedly igneous. The fact that it contains distinct fossils 

 shows that it is fragmental, and it locally becomes coarse, so that its real character is more 

 evident. In thin section under a microscope the fine -fossihferous rock is clearly fragmental, 

 and much of the sediment was evidently derived from the erosion of volcanic rocks. 



The maximum thickness of the Bicknell sandstone is probably over 1,000 feet, 

 but its thickness decreases both north and south of Mount Jura, so as to average 

 less than 500 feet. Hyatt finds some elements of its fauna comparable with the 

 youngest faunas of the Jurassic in Europe but concludes from the evidence of the 

 ammonites that it may be really synchronous with the Callovian. 



The Hinchman sandstone is composed essentially of coarse sandstone with shaly parts and 

 some conglomerate which is generally fine and derived chiefly if not wholly from andesitic rocks. 

 Much of it is decidedly tuffaceous. The most common form is greenish gray and contains 

 darker-colored shaly patches but is not conspicuously fragmental. Examined microscopically 

 it is found to be made up largely of fragments of feldspar, augite crystals, and varying pro- 

 portions of andesitic rock, fragments of which are clearly microporphyritic. The feldspar is 

 generally much altered. Some of the augite is fresh, but most of it is altered to chlorite, giving 

 a greenish color to the rock. It is never coarse Hke volcanic agglomerate, though it is sometimes 

 made up largely of ejected volcanic material, some of which is clearly pumiceous. The grains 

 are rarely well rounded by attrition, though it is evident that they have been transported and 

 loosely aggregated on the sea floor, affording corners and cavities where corals, belenmites, and 

 a number of other marine forms flourished. Remains of these animals are not confined to one 

 hori2:on but are scattered throughout the formation. The conglomerate locally becomes coarse 

 and composed of pebbles, some of which are well rounded; others are angular. The largest 

 pebbles are rarely a foot in diameter. One of the most abundant types is a reddish, decidedly 

 microporphyritic andesite. Others less common are dark and macroporphyritic, with either 

 feldspar or hornblende phenocrysts. Limestone pebbles or nodules occur sporadically, but 

 none could be found with fossils. 



In thickness the Hinchman sandstone appears to range from 50 feet to more 

 than 500 feet. The most characteristic fossils are corals of the genus Stylina, 

 related to species of the Corallian of the Upper Jurassic of Europe. 



The Foreman formation is a succession of shale, sandstone, and conglomerate in which the 

 sediment is for the most part derived from rocks which are not clearly volcanic. The shales are 

 often slaty and locally have "pencil structure" and range in color from dark carbonaceous with 

 traces of leaves through gray, which predominates, to shades and tints of red and yeUow. A 

 remarkable feature of the shales locally is their pencil structure, which is particularly well 

 developed in the summit of Evans Peak. The beds of red and gray sandy shales maldng the 

 summit of the peak break up into small columns, often of lead-pencil size but generally 

 smaller. * * * 



