56 



LA TOUCHE : GEOLOGY OF NORTHERN SHAN STATES. 



miles to the south of Bawdwin, running parallel to the Nam-Tu. 

 It will be dealt with more fully later on, in connection with the 

 description of the lower Palaeozoic rocks (see p. 136). At Bawdwin 

 it runs almost due north and south, and its effects are abundantly 

 noticeable in the twisted and shattered condition of the strata in 

 its vicinity. Unfortunately the country round Bawdwin has not 

 yet been topographically surveyed on a sufficiently large scale to 

 enable the relations of these rocks to the strata under and over- 

 lying them to be clearly made out, and it is as yet uncertain 

 whether they should be classed with the Chaung-Magyi series, or with 

 the lowest group of fossiliferous rocks. 



To the south of Bawdwin these volcanic beds are almost every- 

 where overlapped by Silurian sandstones, and 



Extension to south i _i . . t „ 



and south-east. onl Y a PP ear at one or two P laceS > at Lon §- 



tawktao (F 1) ; further to the south at Kun- 



hawt (E 1), where there is a small exposure of sph&rulitic rhyolite, 



and between the latter place and Hunang (E 2). In the ranges 



east of the plateau they are found along the crest of the Loi-len 



range east of Lashio ; among the hills between Mong-Yai and the 



valley of the Nam-Ha ; and on the west side of Loi Twang. 



Where present, they invariably occur between the Chaung-Magyi 



rocks and the lowest member of the fossiliferous series, but they 



do not form a continuous band, and had evidently either been 



subjected to aerial denudation before the deposition of the latter, 



or are overlapped by higher beds. 



The Bawdwin rhyolites exhibit the usual phenomena observed 

 in acid glassy lavas, such as flow, sphse- 

 acteii rographiCal Chai " rulitic and perlitic structures, corrosion of the 

 quartz phenocrysts, and the like (Plate 6, fig. 1). 

 The groundmass is always cryptocrystalline having probably under- 

 gone a certain amount of devitrification, and sometimes exhibits 

 the peculiar breaking up into irregular areas, alternately light and 

 dark under crossed nicols, known as a " quartz mosaic." 



This structure is exactly the same as that which occurs in some 

 of the Malani rhyolites of Western Rajputana of pre-Vindhyan age ; 

 and in the rocks from both localities, wherever this structure occurs, 

 the quartz phenocrysts are surrounded by a " court " or closed 

 area (Plate 6, fig. 2), the quartz of which is in optical continuity 



1 T. D. LaTouche, Geology of Western Rajputana : Memoirs, Geol. Surv. Ind., Vol. 

 XXXV, Pt. I, p. 83. 



