GEM DISTRICTS OF CEYLON AND BURMA—ADAMS 309 
The limestones, which occur in very thick bands over wide areas 
are white and highly crystalline. Some bands are nearly pure, 
others contain little grains of biotite-pyroxene, graphite, and other 
accessory minerals marking the lines of bedding, and they are again 
identical in appearance with those of the Grenville series in Canada. 
These limestones are in some cases more or less magnesian. ‘The 
metamorphism to which the whole district has been subjected was 
very intense and the limestones are in many places very coarsely 
crystalline. As mentioned, the rubies for which this district is 
renowned have these limestones as their original matrix and are 
more abundant in the coarser grained than in the finer grained 
varieties. 
At one place between Mogok and Sinkwa (on the road to 
Thabeikkyin) there occurs by the roadside, closely associated with 
the limestone, a most interesting occurrence of a nepheline rock of 
the variety known as urtite. It is a rather coarse-grained rock, 
dark in color, and showing an indistinct banding, and is composed 
essentially of greenish-yellow elaeolite and a black aegerine-augite. 
It also contains a considerable amount of primary calcite and re- 
sembles very closely certain varieties of nepheline rocks found 
associated with the Laurentian limestones of the Bancroft district 
of Ontario. Under the microscope the rock is seen to contain as 
accessory constituents a grayish-brown sphene in rather large grains 
inclosed in both the augite and the nepheline, as well as a little 
microcline, apatite, and black iron ore. 
The extent and detailed geological relations of this unusual rock 
could not be determined, but it is very interesting as affording an- 
other instance of the association of nepheline rock with bodies of 
limestone, so strikingly seen in the case of the nepheline rocks of 
eastern Ontario, and in a large proportion of the occurrences of 
similar rocks in other parts of the world. 
The only true granite met with in the district is a great intrusive 
mass which is crossed by the Thabeikkyin-Mogok road and which is 
exposed at intervals from about milepost 44 to milepost 30. It is a 
very even, fine-grained, typical gray granite, which at milepost 40.8 
towers up above the road in beetling crags. Under the microscope it 
is seen to be composed of orthoclase with some biotite, and quite sub- 
ordinate amounts of quartz and plagioclase with a few grains of 
rutile. Its contact with the sedimentary series, through which it cuts, 
is not seen on the road as low land conceals it on either side. 
Just east of Sakangei, cutting this granite mass near its eastern 
margin, is an enormous pegmatite dyke which was opened up and 
worked extensively by the Burma Ruby Mines (Ltd.) some years 
ago. ‘The dyke is at least 100 feet wide, although only one wall is 
20837—27——-21 
