ALLUVIAL CONES— THEIR STRUCTUEE. 



221 



many shiftings, the stream will have swept over a whole semicircle with 

 approximately equal and uniform results. 



The formation thus built up is an "Alluvial Cone." As we travel over 

 these cones their forms are usually recognized by the e}'e, though some- 

 times with difficulty. The slant of the cone (of which more will bo said 

 hereafter) is usually quite small, though sometimes very conspicuous. It 

 varies greatly but not capriciously, depending much upon the nature of the 

 materials of which it is composed. Most frequently these cones are so large 

 and so flat, that it is only by very close scrutiny and comparison with sur- 

 rounding objects that their forms are optically recognized, and many cases 

 occur where we become aware of their true figures and relations only by the 

 use of our pocket instruments. There is one feature which the eye seldom 

 recognizes or even suspects. The profiles are not (even typically) truly con- 

 ical, but are slightly curved instead of having a rectilinear slope. They are 

 concave upwards, the slope being a little greater near the apex and slightly 

 or sometimes notably diminishing towards the periphery. The slopes near 

 the circumference usually lie between 1° and 2°; those near the apex 

 between 2° and 3.^°. The lengths of the radii of the bases often exceed 

 3 miles, sometimes exceed '4 miles, and seldom fall below 2 miles. Per- 

 haps 3 miles would be a fair average for those found in the valleys of the 

 District of the High Plateaus. So nearly together are the gateways along 

 the mountain and plateau flanks, each having its own alluvial cone, that 

 the cones are confluent laterally ; giving rise to a continuous marginal belt 

 along the base of the plateau flanks consisting of alluvial slopes which are 

 sensibly nearly uniform. 



The conical form of these accumulations is ordinarily tolerably accu- 

 rate and often remarkably perfect. It is a surprisingly harmonious result 

 of a process which in its elements is apparently irregular, but becomes 

 regular only by averaging the results of its constituents. Not only is the 

 regularity seen in the external form of the cone, but it is found whenever 

 an opportunity occurs to examine its interior structure. This is sometimes 

 revealed to us. In the vicissitudes to which a stream so conditioned is 

 subject it occasionally happens that indirect causes have set it at work 

 cutting into its cone; dissecting it, so to speak, by a deep cut and laying 



