RECENT : SURFACE CLAYS. 



323 



where, however, it is usually confined to hollows in the surface 

 and to caverns. 



The thickness of the red clay sometimes assumes large propor- 

 tions, as may be seen in many of the cuttings 



Thickness • 



along the railway, where it is seen to rest upon 

 a deeply eroded surface of the limestone, often filling ' pipes ' 

 in the latter. Many of these cuttings show that the clay may be 

 20, 30, or even more feet in depth. If the whole of this were to 

 . . represent the residue remaining from the 



solution of the limestone, it would mean the 

 removal of an enormous mass of rock, for the proportion of insol- 

 uble matter contained in the limestone is very minute ; but though 

 much of it must be attributed to this origin, its growth must have 

 been largely supplemented by the denudation of the bands of clay 

 that are interstratified with the limestone. 



An account of the red clay, and of some of the phenomena con- 

 nected with it, was communicated to the Asia- 

 Cen. Collett s t eory. ^ g oc j e ^.y 0 £ g en g a l D y Brig.-Gen. EL. Collett 



in 1888, 1 and should be mentioned here. Gen. Collett thinks that 

 the red clay once covered the whole of the plateau to a depth 

 of several hundred feet ; that it may have been of marine or 

 lacustrine origin ; and that the funnel shaped hollows, to which 

 I have already alluded (pp. 23, 194), are due to the washing down 

 of the clay through subterranean channels in the limestone beneath. 

 But if the sides of the hollows are examined, it will be found that 

 they are really composed of solid limestone, though the rock is 

 often masked by a thick covering of the clay. Moreover, the aver- 

 age thickness of the clay is no greater in the valleys than it is 

 upon the crests of the highest ridges, as it should be on Gen. 

 Collett's hypothesis. And in addition to this, there is no evidence 

 whatever that within recent times the whole of the plateau has 

 been covered by the waters of an ocean or large lake, in which the 

 clay might have been deposited. 



The red clay forms a somewhat sterile soil, so far as the pro- 

 f duction of cereals is concerned, and is parti- 



cularly unfavourable to the growth of wheat, 

 all experiments in this direction having met with failure. It is 

 largely cultivated, however, under the Shan system known as Hai 



1 Some features in the Geological Structure of the Myelat District of the Southern 

 Shan States ; Journ. As. Soc. Bewjal, Vol. LVII, Ft. 2, p. 384. 



Y 2 



