182 BROWN $ HERON: GEOLOGY AND ORE DEPOSITS OF TAVOY. 



andalusite, Billimanite and pyrite, with finely divided graphite. 

 There are here three parallel bands intercalated in the normal 

 argillites, traceable to the north-west as far as the Hcrmyingyi 

 road, and a thinner bed. on the strike continuation, near Kadu- 

 taung (Doodaung) and Kanbauk. Similar rocks are extensively 

 developed in the upper Zinba valley between the stream and the 

 frontier of Siam. It is noteworthy that these rocks do not acquire 

 on weathering the reddish-brown colour of the decomposed normal 

 argillites but remain black and give rise to a blackish soil. 



Types which can best be described as " greywackes " or fine 



agglomerates are next in importance to the 



amlomeratee. *** ° L ^ ne ar o^itcs, predominating over them in the 



country extending from Tavov town to the 

 Mintha granite intrusion in the south of the district, and 

 lying between the estuary of the Tavov river and the Paungdaw 

 granite area. They are also exhibited at intervals in the gorge 

 of the Great Tenasserim river where it breaks across the strike 

 of the ranges between Myitta and Sinbyudaing. These appear to 

 be the rocks described by Dr. Oldham as u pseudo-porphyritic 

 grits and conglomerates." 



They are dark grey, almost black, when fresh, weathering to an 

 ashy or brown colour, and consist of a confused and structureless 

 jumble of small angular fragments of fine-grained rocks in a fine 

 matrix identical with the material of the argillites. 



The fragments are of quartz, slate, tine quartzite, felspar, etc., 

 and are normally in size smaller than a grain of wheat, with an 

 occasional one as large as a hazel nut. and a very few rounded pieces 

 of granite about three inches in diameter were seen. Incidentally 

 the presence of the last mentioned proves the existence of a granite 

 older than the Mergui Series, which has up to the present not been 

 found. In hand specimens and in the field these rocks strongly 

 resemble volcanic tuffs, and as coarse agglomerates of indubitable 

 volcanic origin have been found in the Mergui Archipelago, they 

 must be the finer ejectamenta of paroxysmal eruptions. They 

 are extremely hard and tough, are unaltered except for indu- 

 ration, and show no stratification nor cleavage. 



Systems of almost vertical joints, (> inches to 3 feet apart 

 traverse them fairly regularly, and there are also less definite vertical 

 cross- joints, perpendicular to these, and horizontal joints. Amongst 



