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 “ 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, 
