54 REPORT OF THE SECRETARY. 



commences near the middle of the region, with a thickness of at least 

 2,300 feet, and diminishes southward to 700 or 800 feet. It is composed 

 wholly of igneous fragments often of great size, inclosed in a matrix of 

 sand and clay. In the southern portion it is underlaid by red sand- 

 stone and white marl of Tertiary Age ; in the middle portion, by rocks 

 of the porphyritic class. The northern and higher portion consists of 

 well stratified rocks, having the mineral characteristics in some places 

 of trachyte, in others of rhyolite, but with a structural habit, a texture 

 and general mode of occurrence very unusual in that kind of rock. 

 Lava-beds occur abundantly, but, so far as observed, are restricted to 

 the foot-hills and valleys, and have evidently originated from the vicin- 

 ity of the great faults. 



Captain Button is also engaged in the investigation of the micro- 

 structure of the rocks of that region, and has made considerable prog- 

 ress in the preparation of specimens for microscopic examination. 



Prof. O. A. White, paleontologist of the division, visited many points 

 in Northern Utah, a few in Northwestern Colorado, and a few in South- 

 ern Wyoming, making a re-examination of the sedimentary beds of 

 that region, and the evidences upon which they had been previously 

 separated into groups. Many localities where fossils had been obtained 

 in previous years were visited, and other localities were discovered. 

 The collections made were from the upper portions of the Carbonifer- 

 ous series through the whole series to near the summit of the Tertiary. 

 The collections were chiefly of invertebrate fossils and are very full and 

 satisfactory. Many new species, and also several types hitherto un- 

 known in American strata, have been obtained. It is an interesting 

 fact, also, that while the change from a salt or brackish water condition 

 of the earlier Tertiary deposits to a wholly fresh-water condition took 

 place without producing any perceptible physical change in the charac- 

 ter of the strata, the species, mostly molluscan, were more numerous, 

 and the differentiation of types much greater during the prevalence 

 of salt in the water than at any subsequent time after the waters be- 

 came wholly fresh. Indeed, in all the purely fresh-water strata of the 

 Tertiary groups the species and genera are few, and there is a remark- 

 able uniformity of type throughout. Both brauchiferous and pulmonate 

 mollusks range through all the Tertiary strata, except that thus far 

 none of the latter have been found associated with brackish- water forms. 

 The Green River group has furnished several species of insects. Of 

 vertebrate remains, some massive fragments of bones of a very large 

 saurian, found in Jurassic strata five miles west of Vermilion Caiiou, 

 are worthy of remark. Scales and detached bones of teleost fishes were 

 found in considerable abundance in dark shales at the very base of the 

 Cretaceous groups at Vermilion Canon, and also at various other points 

 at the same horizon. Teleost fish remains, mostly very perfect, were 

 obtained from the Green River group, and also some from other Ter- 

 tiary strata. Throughout the whole Tertiary series more or less verte- 



