74 GEOLOGY OF TEE 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. Conglomerates. — The coarser clastic formations greatly surpass 

 the tufaceous beds in bulk. Tliey are also much more variable in their 

 modes of stratification and mechanical texture and present problems of 

 great interest. 



1st Texture. — 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: — 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 



