GEM DISTRICTS OF CEYLON AND BURMA—ADAMS 299 
frequently being separated by a transitional layer only a few 
tenths of an inch thick. Thus the clay when washed away by the 
tropical rains or cut through in road making lays bare surfaces 
of clean fresh gneiss, which under the microscope is seen to show 
no traces of alteration. It is thus possible to see good exposures 
of the underlying rock, at intervals at least, in almost all parts 
of this Archean area. 
The heavy rainfall on the island runs off these high lands in a 
system of streams coming together into small rivers. These occupy 
deep V-shaped valleys whose course is usually determined by the strike 
of the gneissic rock, but in some cases follows the direction of joint 
planes or lines of faulting which cross the strike of the rock at 
right angles. The bottoms of these valleys are occupied by heavy 
alluvial deposits laid down by their respective streams, and it is 
in these alluvial deposits that the gems are found. 
The gems have, of course, in all cases been derived primarily 
from the ancient Archean rocks which underlie the whole country, 
but they are seldom found in these rocks. John Davy, M. D.,? who 
visited the island in 1818, in a letter to his brother, Sir Humphrey 
Davy, written in that year, says, “I have ascertained that the native 
rock of the sapphire, ruby, cats-eye, and the different varieties of 
zircon is gneiss. ‘These minerals and cinnamon stone occur em- 
bedded in this rock.” A. R. Coomaraswamy, however, who for a 
number of years was Government mineralogist of Ceylon and is 
one of the most trustworthy writers on the mineralogy and geology 
of the island, in a paper written some years ago says that most of 
the interesting gems of Ceylon have not as yet been found in their 
original matrix. 
J. S. Coates, Esq., B. A., the present Government mineralogist, in 
whose company the writer had the pleasure of visiting the gem work- 
ings in the Ratnapura district, informed him that he believes the 
various forms of corundum (sapphire, ete.) originate in quartz-free 
pegmatites cutting the gneissic series. If such proves to be the case, 
the occurrence is essentially identical with that of the corundum in 
the Bancroft district of Ontario.2 The beryls (aquamarines) the 
writer has himself seen in quartz pegmatites, and Mr. Coates states 
that the zircons have their origin in the same rock. In Burma the 
rubies undoubtedly originated in the limestone bands of the gneissic 
series. Davy’s statement may have been based on some information 
given to him by the natives, or by the term “gneiss” he may and 
probably did mean the gneissic series as a whole. 
2 Journal of Science and the Arts, Vol. V, 1818, p. 233. 
® Adams and Barlow, Geology of the Haliburton and Bancroft Areas, Province of On. 
tario, Memoir 6, Geological Survey of Canada, 1919, p. 327. 
