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MR. C. BARRINGTON" BROWN AND PROFESSOR .1. W. JEDD 
2. Acid Rocks. — “Pegmatites’' and “ Aplites.” 
These rocks, which occur as bands in the gneiss, consist mainly of an alkali-felspar 
with more or less quartz and certain accessory minerals. Mr. Barrington Brown's 
study of these Burma rocks in the field has led him to the same conclusion as that 
arrived at by Mr. F. B. Mallet in mapping the gneissic series in South Rewah *— 
namely, that these acid rocks are not granitic intrusions, but that they constitute an 
integral part of the mass of crystalline schists. Very similar rocks occur in Ceylon 
and in the Salem district, and have been admirably described by M. Alf. Lacroix.! 
We shall, however, in the sequel, have to point out some very interesting points of 
distinction between the Burma pegmatites and aplites, and those of Ceylon and 
Southern India. 
The pegmatites of Burma vary very greatly in their texture. Sometimes the 
individual crystals of felspar are as large as a man’s fist, and occasionally thev attain 
even greater dimensions ; but this coarse rock passes by insensible gradations into 
one of a more finely granular character, to which the name of aplite or granitell 
may be given. When, as is frequently the case, garnets are present, the rock passes 
into a true granulite or leptynite (“ Weissstein ” of German authors). Examples of 
the largely crystalline types (pegmatites) are found at several points about Mogok, at 
the pass leading from that place to Momeit, and many other points in the gneiss 
area; and fragments of them are very widely distributed in the ruby earths. The 
more finely granular varieties appear to have an equally wide distribution; very 
interesting examples of this type are found at Mandalay hill—some of them being 
stained of reddish and greenish tints by alteration products. 
The “pegmatites” often, but not invariably, exhibit traces of the graphic structure, 
and sometimes pass into true graphic granites. The finer grained rocks (“ aplites ”) 
usually exhibit a more or less distinct granulitic structure. 
The proportions of the several mineral constituents in these acid rocks are not less 
variable than their texture and structure. As a rule, the felspar greatly predominates, 
and the quartz and mica are quite subordinate, the latter mineral being not unfrequently 
altogether absent. When the quartz is present in small quantities, it is usually found 
only as scattered grains enclosed in the felspar; when the quartz is present in 
larger quantities, it occurs intergrown with the felspar, to form a true graphic granite. 
Occasionally both quartz and mica are present in considerable quantities, and the 
rock becomes a typical muscovite granite, or rather granitic gneisss. 
The characters presented by the constituent minerals of these rocks are as follows :— 
The felspar is an Orthoclase, which is nearly always of a white colour. Though it 
frequently exhibits, in its undulatory extinction, evidence of having been subjected 
to mechanical stresses, yet it seldom or never exhibits the microcline structure. In 
* ‘ Manual of the Geology of India,’ vol. 1, p. 21. 
f ‘ Bull, de la Soc. Er. de Mineralogie.’ 1889. 
