58 



LA TOUCHE : GEOLOGY OF NORTHERN SHAN STATES. 



for instance, and in the exposure at Wantawng (G 5) on the 

 west side of the Loi Twang range (Reg. Nos. 17/796, 19/966). 1 

 A lava with large spha?rulites is also found on the pass between 

 Nawa (I 3) and Mong Yai, the capital of South Hsenwi, in 

 the ranges south of Loi Ling (Reg. No. 20/957). 



The tufTs form by far the greater part of the bulk of these 



deposits, the flows of rhvolite being quite 



subordinate. They resemble the latter in 

 composition, and consist of fragments of quartz and felspar imbed- 

 ded in a fine-grained matrix of volcanic dust. It is interesting to 

 find that occasionally a fragment of rhyolite occurs in the matrix, 

 showing the mosaic structure of the groundmass referred to above. 

 This observation seems to prove that this structure must have been 

 developed in the lavas at the time of their consolidation, as I have already 

 contended in my account of the Malani rhyolites (Loc. cit., pp. 84, 

 89), and is not due to devitrification of the groundmass at a 

 later period. In this contention I am supported by Dr. Fermor, 

 who came to the same general conclusion from his study of the 

 lavas of Pavagad Hill (Op. cit., p. 163), though he is inclined to 

 think, arguing from the case of a granite from Singbhum, in which 

 a similar mosaic occurs, that the development of the structure 

 was not absolutely contemporaneous with the primary solidification 

 of the rock, but took place immediately afterwards, while the mass 

 was still hot, and is to that extent of a secondary nature. 



The felspar of the tuffs is generally in as decomposed a condi- 

 tion as that of the lavas, and is replaced in the same way by sul- 

 phide ores in the neighbourhood of the great Bawdwin overthrust. 



It seems to be a reasonable hypothesis that the Chaung-Magyi 



series and the older crystalline rocks formed in 

 acJjviiy d of volcanic early Palaeozoic times a land surface, perhaps 



with a cluster of detached islands or shoals to 

 the south-east of the main area, the shores of which were washed by 

 the seas in which the succeeding fossiliferous strata were deposited. 

 These older rocks had been folded, upheaved, and greatly denuded 

 before this deposition occurred, and it seems not unlikely that, at 

 some time during this period of disturbance, volcanic forces became 

 active, and, if so, that the most probable places in which the out- 

 breaks would occur would be situated along the shores of this 



1 These numbers refer to the register of rock specimens in the Geological Survey 

 Museum. 



