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Scientific Proceedings, Royal Dublin Society. 



longer appears to consist almost entirely of individual grains of 

 quartz, but of a few larger and more conspicuous ones imbedded in 

 a fine-grained matrix. The greater visibility of the sericite due to 

 its polarizing in brilliant colours does not wholly account for this, 

 much more is it due to an optical heterogeneousness in the appa- 

 rently homogeneous quartz grains ; some of the smaller of these 

 are resolved into a mosaic, the separate fragments of which are 

 mortised into one another, while the larger are scalloped round 

 the edge with inlet granules of different optical orientation. The 

 whole appearance is similar to that with which we are familiar in 

 the crushed quartz of the foliated granite of the district, and there 

 can be little doubt that under the pressure to which the Cambrian 

 rocks have been more than once subjected, very considerable 

 crushing has taken place, breaking many of the smaller grains to 



Fig. 2. — Section of a grain of quartz from the quartzite of Carrickgologan. Seen with 

 polarised light, and highly magnified. F". V.' lines of vapour cavities. The areas 

 extinguished in the upper and lower parts of the section are not so clearly arranged 

 in hands in this as in some other specimens. 



powder and detaching parts of the exterior from the larger : a 

 cementation of the fragments has followed upon the crushing, restor- 

 ing to the several grains their original continuity. The pressure 

 which thus transformed the original material of the rock has left 

 other traces of its action ; the unbroken mass of the larger grains 

 seldom extinguishes between crossed Nicols like the quartz of an 



side by side for comparison, but unfortunately are differently orientated : the elongated 

 rounded fragment in the S.E. corner of fig. 2, and the N. W. of fig. 1, is the same in 

 both, from which it will be seen that a rotation of 180" in the plane of the paper will 

 bring fig. 2 into a corresponding position to fig. I. 



