448 PROCEEDINGS OE THE GEOLOGICAL SOCIETY. [ilay 28, 



in section ; for when a gully has cut through the accumulation in a 

 radial direction, it shows what is drawn in fig. 5, straight-sloping 



Fig. 5. — Radial Section of a Fan, 



parallel layers ; but if a section has been made by the main-valley 

 stream across a portion of the fan, we see the lines of accumulation 

 to be in curves, as shown in the diagram, fig. 6. These curves must 



Fig. 6. — Section -of a Fan on a clwrcl. 



be, from theory, hyperbolas. The lateral changes of position of the 

 depositing stream, and the partial growth of each layer, are denoted 

 by false-bedding. 



It may be remarked that the only difference of form between an 

 alluvial fan such as this and the fan talus sketched in fig. 2 is in the 

 degree of slope. 



An unusual case of a fan was observed at Murgi, in ]S"ubra, where 

 the side-stream entered the main valley as a waterfall with a height 

 of about 200 feet. Where this fall was, the fan had its apex ; and 

 from that point (on which, of course, the debris, as well as the water 

 itself, fell) it spread just like any other. 



The slope of the alluvial fans varies. I have noted the inclination 

 of some fans near Thonde, in Zanskar : four in succession had an 

 angle of 6°, 4°, 6°, and 8° ; again, opposite to Leh, the angles 3°, 

 4°, and 6° occur. It is the case that the smaller fans have the 

 greater slope, and the larger ones the more gentle slope; again, those 

 have a gentle slope that belong to, and lie in front of, ravines 

 that reach far back into the mountains. This must be because 

 such ravines have streams which, draining a larger area, or 

 coming more from snow and glaciers, carry a greater volume of 

 water ; and with that greater amount, and consequent greater car- 

 rying-power, of the water, the detritus will not remain on a slope of 

 the higher degree, but will only be deposited when it can form for 

 itself a cone or fan of the slope corresponding to the circumstances 

 of the character of material and transporting-power. 



In the Nubra valley, from which I have taken my example, the 

 fans commonly are distinct and separate, with a space of main-river 

 alluvium between each ; but it is perhaps as often the case that they 

 have been formed so near to each other as to meet in the course of 

 their growth and to touch each other ; and in such cases the layers of 

 each must have slightly overlapped in the forming. 



On the left bank of the Indus, opposite Leh, there is a great ex- 

 ample of this amalgamation of fans. There, from a succession of ra- 



