424 Transactions of the 



the shortest course — thus distributing it with equality to all parts of 

 the intervening surface. To do this the irrigating furrows will main- 

 tain a direction generally perpendicular to the curve of the ditch, 

 diverging from eaeli other where the curve is convex, and converging 

 where it is concave. The lines of checks from furrow to furrow will 

 themselves he curved and substantially parallel with the curve of the 

 ditch. Before, therefore, the farmer can proceed intelligently to make 

 his irrigating furrows, the engineer must run him a set of contour lines 

 sufficiently close to enable him to determine the direction of inclination 

 in each field. These matters once ascertained, and marked permanently 

 upon the ground, the detail of irrigating these " uneven " lands will not 

 differ in practice from that applicable to the true inclined plane surface 

 of the " west side." 



FORMATION OF THESE SOILS. 



Without going profoundly into the geology of these regions, the 

 method of the formation of soils differing thus widely in character hav- 

 ing apparently 011I3* one characteristic of unbounded fertility in com- 

 mon, can be explained. The hog-wallow, with its underlying hard-pan, 

 shows the retreat of the broad foot of the glacier, leaving behind it the 

 accumulated dust and debris of pulverized rock, arranged in its present 

 form by the innumerable rills that issued from the retiring sheet of ice. 

 The same rocks were ground up to produce the elements of the loam 

 and of the adobe; we find only that these elements subsist in the tAvo 

 soils in different states of mechanical division. In the loams deposited 

 by waters flowing in streams and rivers with a velocity sufficient to 

 carry along much of the matter suspended in a state of extremely 

 minute division, we find the constituent particles of coarser grain. It 

 would be difficult to believe that the vast areas of adobe constituting a 

 large part of these plains, and often of great depth, were due to a sur- 

 face wash of water like that of ordinary rainfall. But directly along 

 the base of the foothills, overlying perhaps the hog-wallow, or the 

 loam of the original formation, we find a strip of true adobe, from a 

 half mile to five or six miles wide, bearing the color of the hillside 

 above it, which appears evidently to have been formed by rain- wash 

 from the hill above. The effect might still appear too great to be 

 attributable to this cause, till we learn that a considerable part of this 

 strip of adobe — in some localities known as "dry bog" — has increased 

 or been formed within a few years past; and a new conception of the en- 

 ergy of this cause comes with the knowledge that of one strip, five miles 

 in width, more than half was formed, over what had been hog- wallow, 

 during a single wet Winter — that of eighteen hundred and sixty-four. 

 Here we learn that the rain-wash does actually form adobe; that its 

 energy is sufficient to form an extensive area of it in a singlo season; 

 and accept without difficulty the conclusion that the same soil has been 

 formed elsewhere by the same agency. This explanation at the same 

 time accounts for the peculiarities of the adobe, the absence of grit in 

 its composition, its difference from the loams, and the relative position 

 of the two where they are found in juxtaposition. 



VALUE OF DRAINAGE. 



Knowing the area which drains into a watercourse and the amount 

 of rainfall over that area, the volume of water that will be discharged 



