16 
Bulletin 65. 
been transported from the mountains by the streams or waters 
which have given the country its present contour, but are disinte¬ 
gration products of formations composed of material similar to that 
brought from the mountains by the streams of later times. 
The particles of more than 0.5 millimeters diameter are for the 
most part rounded, but this is not the case with those less than .25 
millimeters, nearly all of which are sharply angular. 
§ 33. It may appear to many, especially to such as have an 
acquaintance with the occurrences of loess and other formations of 
the plains, the mineralogical constituents of which are often identical 
with those given for these soils, that the latter, like the former, may* 
be the product of other disintegrations than that of the granites of 
the Colorado range. There is nothing in their composition to pre¬ 
clude their having been derived from some other source. Their 
geographical distribution makes it probable that material similar 
to this composing our soils has been derived from other mountains 
than those lying near to us. It seems reasonable, however, to 
assume that the near and very extended range has furnished 
essentially all of the material going to make up these soils. There 
may be a doubt entertained that the quartz, feldspar and mica 
given in the preceding table were derived from the present moun¬ 
tains, but there can be no question regarding the source of the frag¬ 
ments of red and brown sandstones, or of the impure limestone; 
these are the products of the disintegration of the red beds and of 
the Fort Benton limestone. 
SOIL COMPARED WITH LOESSIAL SOIL. 
§ 34. There is given in the table of mechanical analyses, one 
of a loessial soil from Weld county. This soil, according to the 
mechanical analysis, has a very different composition from the soils 
given above. It is characterized by a high percentage of fine sand 
and low percentages of dust and clay, but mineralogically the sand 
and coarser parts are the same as in the other soils, i. e., quartz, 
feldspar and mica. The mica, though still subordinate in quantity, 
is more abundant in the loessial soil than in the others. It seems 
to be a general rule that the farther back from the river we get, or 
higher up on the plains, the greater is the percentage of fine sand 
present in the soil. 
§ 35. I have not attempted to determine the ratio between 
the grains of feldspar and quartz in these soils, but others have 
determined this ratio in the loess, for which the approximate deter¬ 
mination is given as : Quartz, 40 per cent.; feldspar, 50 per cent.; 
other constituents, 10 per cent.* The minerals making up the 
other 10 per cent, were, in the case of the Weld county loessial soil, 
* Emmons’ Geology of the Denver Basin in Colorado, p. 262. 
