OCR Text |
Show 105 Utah, of the Cretaceous age of the coals of Castle Valley and Southern Utah, and of the Cretaceous age of the disputed coal- series of Colorado. Tertiary, Cretaceous, Jurassic, Carboniferous, and Silurian rocks have been identified by fossils in Utah, and all of these but the Jurassic in New Mexico; in Colorado and Arizona, Cretaceous, Carboniferous, and Silurian; in Nevada, Jurassic, Carboniferous, and Silurian; and in Southern California, Carboniferous and Silurian. The age ( heretofore in doubt) of the shale and sandstone at the base of the series of strata exposed in the Grand Canon of the Colorado has been ascertained to be' primordial. Of special subjects of study, one of the most interesting has been that of the glacial epoch. The southern limit of the ancient system of glaciers has been ascertained through the entire extent in longitude of the survey, and an attentive examination hasjbeen made of the record of an expansion of Great Salt Lake, which occupied the valleys of Utah, while its highest mountain- gorges were choked with ice. The elaboration of these results into reports for publication has occupied the geologists during the winter- months. The report of Mr. Marvine is in manuscript, ready for the press, and the same may be said of the report of Mr. Gilbert for the field- work of the seasons of 1871 and 1872, and of thatof Mr. Howell. A portion, however, of the notes of the latter gentleman have been put into the hands of Mr. Gilbert to combine with some closely- related examinations of his own, and embody in the report, upon which he is now engaged, of his examinations in 1873. Professor Stevenson has completed and submitted his report, with the 1 exception of a single chapter. The whole will fill, when printed, about 350 quarto pages. ! In the preparation of these reports, the itinerary form, so easy to write, but so inconvenient for use, either by the general reader or by the future student in the same field, has been avoided, and all the material presented has been thoroughly classified. Facts of common character have been brought together, and where their importance warranted, have been briefly discussed in their relations to each other and to cognate facts in other fields; and it is believed that the arrangement is such that the various data will be readily accessible to those who shall have occasion to use it. An atlas of geological maps is in preparation, to accompany the reports. The general facts of rock- distribution are to be indicated by colors laid upon the topographical- atlas sheets, and special maps will be constructed to illustrate some other features, such as the distribution of glacial phenomena and of thermal springs. The illustrations with the text will cousist of fourteen plates, derived chiefly from photographs of peculiar geological features, and of a large number of wood- cuts, the major part of which are now drawn. The wood- cuts are chiefly diagrams and sections representing rock-structure, and are strictly explanatory of the text. The geological collections ( without including the fossils and ores) number 2,700 specimens. One of their chief uses has been already subserved in enabling the geologists to study, during the preparation of their reports, peculiarities of texture and composition that could not receive full attention in the field; but a larger portion of them have a permanent value also as material for special lithological study. It is hoped that at some future day the volcanic rocks, which outnumber all others in the collection, can be placed in the hands of a competent specialist for study. |