SoLLAS — On the Structure and Origin of Quartzite Rocks. 177 



uncrushed rock ; undulose extinction is common, but in many 

 cases this is of such a nature that it would be better termed 

 " banded " extinction, since on rotating the stage of the micro- 

 scope not merely one wave of darkness passes over the quartz grain 

 but several separated by intervening bands of light, and remind- 

 ing one, save for the vague indefinite boundaries, of the bands of 

 the synthetic twinning in felspar (fig. 2) . In some cases in addition 

 to this vague banding there is another and more sharply defined 

 running in another direction, the two sets of banding making 

 frequently more or less right angles with each other. 



A good deal of the quartz contains fine hair-like crystals which 

 I think must be rutile ; they possess a high, apparently metallic 

 lustre, and though apparently opaque, j^et when of unusual 

 thickness they give bright colours between crossed Nicols. These 

 prisms evidently grew with the quartz, and they have since shared 

 in its deformation, since in some cases one finds a long prism 

 disjointed into a succession of short lengths which neither touch 

 one another nor lie along the same straight line nor even maintain 

 a perfectly constant direction. 



In addition to quartz an occasional flake of muscovite may be 

 sometimes observed; frequently it seems to have almost entirely 

 escaped the effects of crushing, but now and then the remains of a 

 crystal which has been squeezed to an indistinguishable wirr of 

 fragments is encountered. Yery rarely a stray zircon or a rounded 

 crystal of tourmaline may be identified : felspar appears to be 

 entirely absent. 



Kilrock Quarry, Howth. — The remarkably compact quartzite 

 excavated at this quarry is certainly the one I should be inclined 

 to select as the most likely to show but few traces of its 

 original structure, so homogeneous is it and fine-grained. Thin 

 slices, however, show very much the same features as the pre- 

 ceding rock ; there is, perhaps, not quite so much sericite and 

 somewhat fewer rounded grains of quartz, but the evidence for a 

 clastic origin is just as striking. Planes of vapour cavities tra- 

 verse many of the grains, and sometimes a set of planes may be 

 observed with a common direction, which they may pursue through 

 as many as five or six consecutive grains, thus indicating the for- 

 mation during crushing of cracks extending through the rock, 

 which afterwards healed up. In some cases banded extinction 



