DEVONIAN SYSTEM : PLATEAU LIMESTONE. 



191 



a very minute proportion of calcite only being present, as shown 

 by staining, in the interstices. 



Nos. 15/898 and 17/819, from a low hill near Mongyaw (Lat. 



23 2' : Long. 98° 9', i.e., just beyond the 



Oolitic dolomites. ,. ., , . j , 



northern limits of the map), on the road from 

 Lashio to the Kunlon Ferry, are oolitic dolomites, containing numerous 

 specimens of minute foraminifera, among which 

 a species of Endothyra is the most common, with 

 a few of Textularia and perhaps Trochammina. The groundmas/s 

 of the rock, which consists of a granular mosaic of cloudy dolomite 

 crystals, the oolitic granules, and the organisms are all unaffected by 

 Lemberg's solution, showing that no calcite is present as such in a 

 visible form. In No. 15/898 (Plate 12, fig. 1) the oolite granules 

 are circular or oval in shape, defined by a thin dark line, and 

 freely distributed through the groundmass either singly or in pairs. 

 The interior is lined by an aggregate of dolomite crystals, generally 

 exhibiting idiomorphic outlines, of larger size and usually less cloudy 

 than those forming the groundmass. These sometimes fill the whole 

 of the granule, but in most cases either did not do so originally 

 or have been dissolved or broken away, so that the rock is now 

 filled with minute pores. The organisms lie in the groundmass 

 outside the oolitic grains. No. 17/819, though 



Brew iated Oolites. . .. . . . . . ' 



also an oolitic dolomite, is of a different char- 

 acter (Plate 12, Fig. 2). In it the oolitic granules are coagulated 

 together into large, semi-opaque, irregularly shaped patches, which 

 appear to be crushed fragments of an original oolitic limestone, foi 

 they are cemented together by a fine-grained matrix of granular dolomite 

 which seems to have grown outwards from the edges of the individual 

 patches ; for, wherever these are parallel to each other, the space be- 

 tween is occupied, first by a zone of fine-grained, cloudy dolomite, and 

 then by larger, often idiomorphic crystals of clearer dolomite, filling 

 up the centre of the space after the manner of a mineral vein. 

 In some instances the zoned effect is very clear, the crystals more 

 immediately in contact with the oolitic patches exhibiting a ' dog- 

 tooth ' arrangement, with the apices of the crystals pointing out- 

 wards. Most of these ' dog-tooth ' crystals themselves are zoned 

 with cloudy matter. (This structure is well seen in fig. 1, Plate 13). 

 Single oolitic granules and foraminifera are freely scattered through 

 the matrix. The structure of the granules is concentric, in alternate 

 minutely crystalline and ' dirty ' bends. In some cases the nucleus 



