1876.] 
Geology . 
567 
The “ American journal of Science and Arts ” contains a paper by Mr. 
G. K. Gilbert on “The Colorado Plateau Region considered as a Field for 
Geological Study.” This region is 170,000 square miles in extent, and is 
remarkable for the rocky character of its surface and its extreme aridity. The 
only mineral produff of economic importance is coal, which is worked only 
where the Pacific Railroad affords means of transport. “ The air is so dry 
that, except on the heights and on the margins of springs and streams, there 
is no turf, no accumulation of humus, often no soil, and so little vegetation 
that the view is not obstruffed. From a commanding eminence one may see 
spread before him, like a chart, to be read almost without effort, the structure 
of many miles of country. There is no need to search for exposure where 
everything is exposed. The strata are undisturbed, and are cut by valleys of 
erosion, in the wall-like sides of which every inch of the series may be 
examined.” It is evident that such a region must afford remarkable facilities 
for studying mountain-building by displacement, stratigraphy, and the problem 
of the canons. The author remarks — “ Already the field has yielded to its 
students results new to them, and probably new to the world of science. 
Among them are a type of uplifted mountains, a type of eruptive mountains, 
a theory of water-falls, and a classification of drainage-systems.” 
Under the title of “ The Geology of Portions of our Western Territory,” 
Mr. Gilbert gives the results of an examination of portions of Nevada, Utah, 
California, and Arigona. He treats of the orology of the distriff, of its 
valleys and canons, of the glacial epoch, the water-supply, the volcanic rocks 
and mountains, and of the stratified rocks. Concerning the glaciation, he 
infers that the general glaciation of the Eastern States had no counterpart, in 
the same latitudes over the region extending from the Rocky Mountains to the 
Sierra Nevada inclusive. There were in that region local glaciers high upon 
the flanks of the mountains, the most southerly of which did not extend lower 
than an altitude of 8000 feet above the sea. There was a general accession 
of water to the valleys of the great basin. Lakes were formed where there 
are now only deserts, and valleys now nearly empty were filled to overflowing. 
The flooding of the valleys is correlated in time with the formation of glaciers 
upon the mountain summits on the same principle on which the different local 
floods are correlated with each other, the local glaciers with each other, the 
glaciers of the East with those of the West, and those of America with those 
of Europe, — namely, that the phenomena were of the same class and occurred 
in the same division of geological time. Each took place during the post- 
tertiary, and each marked a climatal change of polar tendencies. The phe- 
nomena of the Glacial epoch at the West differed from the synchronous 
phenomena in the same latitudes at the East, for the reason that then, as now, 
the former region was comparatively arid, and material was lacking for a great 
ice-field. The configuration of continents was not so far different from the 
present but that the principal climatal districts were marked out, and the great 
flexures of the lines limiting zones of climates were arranged very much as 
we now know them.” On the subject of water-supply the author enters upon 
a practical question of great moment, and especially interesting to an empire 
which embraces regions so deficient in rain as are certain parts of Australia 
and South Africa. What is the influence of agriculture upon climate ? Will 
a cultivated country when brought under cultivation induce a larger amount of 
atmospheric precipitation than it did when in the state of a desert , — not, we 
must remember, of forest? Mr. Gilbert admits that the water of the Great 
Salt Lake has risen ever since, or nearly ever since, the occupation and culti- 
vation of the neighbouring country by the Mormon settlers. He admits, 
further, that the rise of the lake indicates an increase of rainfall as compared 
with evaporation, and that large areas of land brought under cultivation are 
cooler than desert-surfaces, and hence better able to induce rainfall from cur- 
rents of moist air. But he maintains that this increased rainfall is due to the 
increase of evaporation brought about by the wide distribution of water in 
irrigation, and that there seems no reason to believe that the result in precipi- 
tation shall exceed — if indeed it can equal — the expenditure in evaporation. 
This remark on the influence of irrigation — a practice very extensively resorted 
