4 The Colorado Experiment Station 
leaching of these nitrates out of the shales will have to account for 
the fact that these overlying mesas are dotted with brown spots, 
rich in nitrates, while there are no shale banks above them from 
which the nitrates may have been leached. 
If we are to consider seriously the shales as the source of these 
nitrates, we are compelled not only to consider them rich enough 
in nitrates to permit of capillarity carrying them to the surface and 
causing their deposition, but we must consider the shales as holding 
a very great store of them, so great that the time and the water 
necessary to erode our valleys has been insufficient to wash 
them out. 
It is well known, that under certain conditions, nitrates may 
occur in soils in sufficient abundance to permit of their crystalliza¬ 
tion. These conditions are, however, by no means very common. 
Such occurrences of nitrates are given in our text books, particularly 
in our mineralogies, and are matters of common knowledge, so much 
so that some such origin would in all cases be the first one sug¬ 
gesting itself for consideration. Concerning the application of 
these facts to the shales as the origin of the nitrates it was plainly 
stated that we did not consider that they played this part for two 
reasons: First because many brown spots occur on the mesas above 
the shales; Second, because the brown spots occur in entirely dif¬ 
ferent geological horizons where the shales do not occur, in alluvial 
deposits and under our ordinary prairie conditions; in other words 
the shales, provided that they contained nitre, could not be con¬ 
sidered as the explanation for the greater number of the occur¬ 
rences and independent of any other reason than their insufficiency, 
we must seek for a more general cause, one sufficient to account 
for all of the occurrences. This assumes that they have a common 
cause, which is a reasonable assumption so long, at least, as we are 
not sure that they actually have several different causes. 
The origin of the alkalies in such countries as ours is beyond 
doubt correctly explained by attributing their formation largely to 
the various changes suffered by the felspars under the action of 
water, more or less strongly charged with carbonic acid, but in the 
absence of a sufficient supply of water to carry away the products 
of their decomposition. This appears to be an entirely adequate 
source to yield the chlorids, carbonates, sulfates, etc., which we 
find in our soils, or present as alkalis, but these are not the source 
of the nitrates. 
I am fully aware that students of geology made record, more 
than twenty-five years ago, of the observation that the Cretaceous 
shales seem everywhere to be charged with alkalis. These alkalis 
are, in some cases, composed wholly of sulfates, in others they are 
