72 GEOLOGY OF THE HIGH PLATEAUS. 



usually occurs either as the magnetic oxide or protoxide. In the protoxide 

 forms it is always in combination in some of the minerals — the undecom- 

 posed hornblendes and micas or such alteration products as epidote or viri- 

 dite. These alteration compounds, particularly, are more or less thoroughly 

 diffused throughout the mass of the rock, impregnating it with a greenish 

 color, while the unchanged mica, hornblende, and magnetites, disseminated 

 as black particles, give the rocks a gray color of varying shades from very 

 dark to very light. Whenever these beds have been subject to metamor- 

 phic action, as has often happened, the proto-compounds of iron are often 

 converted into sesquioxide, producing a pinkish color similar to that of 

 " Scotch granite." Thus the colors of the tufaceous beds would enable us 

 to single them out as presumably composed of materials very different from 

 those constituting ordinary sandstones. 



All of these finer beds are stratified after the manner of ordinary 

 aqueous deposits. That they were water-laid is unquestionable. No rocks 

 have been observed which could possibly have been accumulated by the 

 precipitation of volcanic ashes upon the land. The agency of water in 

 arranging them in their present form is altogether too conspicuous to admit 

 of any doubt. The origin of these clastic materials, proximately considered, 

 is in the break up and destruction of older massive volcanic rocks by the 

 ordinary processes of denudation. It is, indeed, possible that some small 

 proportion of their ingredients may have been pulverulent material blown 

 from volcanic orifices and washed into the basins where the strata accumu- 

 lated, but it seems quite certain that the great bulk of the tufas did not so 

 reach their present positions. They differ in no other material respect from 

 the common lacustrine beds than in the sole fact that they are the debris of 

 volcanic rocks instead of sandstones and gneisses. In a number of instances 

 they are seeu to pass, along horizontal exposures, by a gradual transition, 

 into common lacustrine deposits, the quantity of material derived from the 

 break up of vocanic rocks becoming gradually less and less, while that 

 derived from the disintegration of foliated rocks becomes greater and 

 greater. Instances of this transition are seen in various parts of the Sevier 

 Plateau and in the beds beneath the lava-cap of the Markagunt. Indeed, I 

 doubt not that those beds, which are apparently most typically " tufaceous," 



