326 



RESUME AND THEORETICAL DISCUSSION 









Wards, but with irregularity, the depth to which this change has penetrated 

 varying with the varying character of the conditions. 



That the red gravel is a more or less decomposed variety of the blue is evi- 

 dent not only from what has been stated above, but from the fact so frequently 

 observed, namely, that the latter is rapidly changed on being uncovered so as 

 to become exposed to air and moisture ; and the presence of water in large 

 quantity seems to favor rapid conversion of the blue into the red variety. 

 When the blue gravel is uncovered so as to be exposed to the ordinary 

 atmospheric changes, even without access of running water, it almost always 

 becomes gradually softened and reddened. Indeed, it would appear to be 

 true, as a general rule, that exposure converts the blue gravel into the red 

 variety, although in some cases much more rapidly than in others. The 

 inference, then, is that all or a large portion of the gravel has been originally 

 blue, and that such portions as have become favorably situated, owing to 

 mining operations or erosion produced by natural agencies, for the action of 

 air and moisture, have been decomposed and oxidized. 



This condition of things presupposes that all, or at least a large part of 

 the gravel was originally in the condition indicated by the word u blue," 

 Such is not usually the case, however, with detrital accumulations of a super- 

 ficial character; and it becomes necessary to explain how it is that the 

 gravel deposits on the west slope of the Sierra became so thoroughly consoli- 

 dated as it is supposed they must have been, judging from the condition of 

 certain portions still remaining unchanged in the deeper channels, as already 

 described. 



■ 



Detrital accumulations belonging to the older formations are usually very 

 solidly compacted together, and there are not many exceptions to this. 

 Occasionally, as in the case of portions of the St. Peter's sandstone in the 

 Upper Mississippi Valley, the grains of which the rock is composed have so 

 little coherence that they can be separated from each other by rubbing in 

 the fingers. The causes of the compacting of the older rocks are apparently 

 considerably varied in character, and, so far as the writer is aware, have not 

 been very satisfactorily studied out. We know, however, that very durable 

 and hard conglomerates and sandstones may be either of red or gray or 

 bluish-gray color, the red varieties being supposed to contain a considerable 

 quantity of iron in the state of peroxide, this forming a portion, at least, of 

 the cement which holds the constituent fragments together. It is evident, 

 however, that the newer the formation in which any detrital rock occurs, 





