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Show 74 GEOLOGY OP THE HIGH PLATEAUS. great lake had receded. Either view is for the present tenable. The small extent of the individual beds might argue for local lakelets. There is no persistent formation subsequent to the Bitter Creek spreading over the entire area of the district, but merely considerable patches of tufaceous beds from 100 to 250 feet thick, having no discovered connection with each other, but occurring in many localities. We find reason for presuming some to be much more recent than others, for they rest upon volcanic sheets or conglomerates which can scarcely be so ancient as the middle Miocene. Those, however, which rest upon sedimentary beds are probably of middle Eocene age, or thereabout, in the southern part of the district, and a little more recent in the northern part of it. No distinguishable fossils have yet been discovered in any of them. On the view that these beds are the waste of older eruptive rocks, the opening of the volcanic activity of the district is thus carried back into the middle or early Eocene. II. Conglomeeates.â€"The coarser clastic formations greatly surpass the tufaceous beds in bulk. They are also much more variable in their modes of stratification and mechanical texture and present problems of great interest. 1st Texture.â€"Like all conglomerates, they consist of rocky fragments inclosed in a matrix of finer stuff, and both fragments and matrix are volcanic material, without any admixture of debris from ordinary sedimentary and metamorphic rocks. The included fragments range in size from mere grains to blocks weighing several tons. They are of the same petrographic characters as the massive rocks of the neighborhood, and side by side lie pieces derived from widely distinct kinds of lava:â€"many varieties of rock may be gathered from a few cubic yards of the same conglomeritic mass. Cases occur, however, where for considerable distances along a given stratum the fragments are all of the same variety; in some the varieties are many; in others they are few. There is no constancy of ratio between the quantity of rocky fragments and the sandy or impalpable matrix. In some beds the stony fragments form but a very small proportion of the bulk; in others, the reverse is true: and there is every possible intermediate proportion. The individual beds are usually very heavy and thick, the partings being rare In many cases the dimensions of the stones are |
