SoLLAS — On the Structure and Origin of Quarfzite Mocks. 185 



and where it frequently is a matter of doubt whether a series of 

 boulders are ancient beach pebbles or merely squeezed out joint 

 blocks of an adjoining bed. 



At first acquaintance these dislocations are not a little astonish- 

 ing, but on reflection one is rather inclined to wonder why they 

 are not of more general occurrence. When a heterogeneous mass 

 of soft shales and hardened sandstones or limestones of varying 

 thickness is subjected to powerful earth-thrusts, one would scarcely 

 expect it to behave precisely as shown by our neat little working 

 models. One of the most general effects of such mountain pressure 

 is the compression of the rock subjected to it in the direction of 

 pressure and its extension in a plane at right angles to it. In a 

 comparatively hard and rigid material the extension is frequently 

 accomplished by fissures (as already shown by Heim and Daubree). 

 Such fissures are sufficiently common in the rocks of our district, both 

 on the large and small scale. I have already described in another 

 place the numerous fine parallel cracks which traverse the felspar 

 of the foliated granite so common on the flanks of the Leinster 

 hills. Exquisite examples of similar fractures are presented by the 

 garnets so common in the mica-schists which result from contact 

 alteration of the sandy slates of the district. The cracks run at 

 right angles to the plane of foliation, and extend from the garnets 

 into the surrounding grains of quartz as planes of vapour cavities 

 (PL XV., fig. 3), which, therefore, are of secondary origin, as Julien 

 has shown in other cases. A vast number of parallel planes of 

 vapour cavities coexist with the preceding, traversing the quartz 

 grains with wonderful continuity, a single vapour cavity plane 

 crossing as many as fifteen to twenty individual quartz grains ; 

 in addition there is present a second system of similar planes, 

 but situated in the plane of schistosity. These are not so readily 

 accounted for ; particularly since they neither shift nor are shifted 

 by the preceding set which they intersect at right angles. 



Passing from cracks of such minute dimensions, attention may 

 next be directed to the specimen represented by the figure (PI. xv., 

 .fig. 5) which represents a bed of quartzite an inch or two in 

 thickness with some associated slate ; the slate shows fairly good 

 cleavage, but the quartzite is traversed by numerous open cracks. 

 Similar cracks extend in great numbers side by side through 

 quartzite beds several feet in thickness, and when filled up with 



