60 LA TOUCHE : GEOLOGY OF NORTHERN SHAN STATES. 



represented by the fossiliferous Palaeozoic and Mesozoic strata volcanic 

 activity of every kind seems to have been in abeyance, and it is not 

 until we reach quite late Tertiary times that we find evidences of 

 such forces being still in existence beneath the surface. Such basic 

 intrusions as do occur are confined to the oldest rocks, and are of 

 rare occurrence in them. One small dyke was 



Diorite near Hpalam. . . , . ... , . 



met with among the mica schists to the south 



of the Ruby Mines, between the villages of Hpalam (C 1) and 



Nalaw, 13 miles east-south-east of Mogok. This rock (Reg. No. 



15/966) is a dark greenish diorite with a specific gravity of 



2*873, consisting of plagioclase felspar (labradorite), and pale green 



hornblende. The latter forms a minutely crystalline cataclastic 



groundmass ophitically surrounding the felspar (Plate 7, fig. 1). The 



only accessory minerals are scattered granules of iron ore, the 



surface of which shows by reflected light the white meshwork 



characteristic of leucoxene, and a few very minute needles of 



apatite. The whole rock is remarkably fresh looking, and the 



hornblende may be of secondary origin, in which case it should be 



styled an epidiorite. The rock differs entirely from the Tertiary 



dykes of Burma in containing no olivine, and I am not aware of 



any other locality in which a similar rock occurs in this region. 



The only other locality in which basic dykes, older than the 



. , , Tertiary period, have been found is in the 



OliYine-gabbro. . , , " ~ , , ■ T , TT 



neighbourhood of Nam Hsan, near the eastern 



edge of the granite area, into which they have been intruded. 



This rock (Reg. Nos. 17/792, 20/559) is quite different from that 



described above. It is a coarse-grained, dense, black, holocrystalline 



rock with a specific gravity of from 3 "012 to 3 '04. The constituents 



are — olivine in large granules with a peculiar violet-brown tinge ; 



plagioclase felspar, either a basic variety of andesine, or labradorite ; 



and colourless augite, the latter ophitically surrounding the olivine 



and felspar. The rock is interesting on account of its resemblance 



in some respects to the olivine norite from a dyke in South Rewa 



collected by Mr. Datta and described by Sir T. Holland in the 



Records, Geol. Survey of India. (Vol. XXX, Pt. I, p. 20), although the 



Nam Hsan rock does not contain enstatite, and cannot therefore be 



called a norite. The resemblance lies in the similarity in the colour 



of the olivine phenocrysts, which is perhaps due to the presence 



of manganese, and in the development of a narrow " reaction rim " 



surrounding the olivine crystals, wherever they are in contact with 



