Vol. 60.] QTJARTZITE-DYKES IN MOTTNTAIN-LIMESTOKE. 369 



Plate XXXI. 



[The figures were photographed by the Author from the microscope, under 

 polarized light with crossed nicols, and enlarged 50 diameters.] 



Fig. 1. Thin slice (1316) from the 4-inch dyke shown in Pi. XXX, fig. 1. 

 See p. 305. 



2. Thin slice (1318), showing the formation of secondary silica in optical 



continuity with the rounded quartz-grains, from a second dyke. See 

 p. 365. 



3. Thin slice (1235), quartz and felspar cemented by calcite, from a third 



dyke. See p. 365. 



4. Thin slice (1086), quartz and felspar, from a fourth dyke. See p. 365. 



5. Thin slice (1238) from Marston-Common well : quartzite. A piece of 



mica is seen near the centre of the figure. See p. 368. 

 0. Thin slice (1319) from the quarry south of Marston-Common Farm : 

 sandstone. See p. 368. 



Discussion. 



The Chairman (Mr. H. B. Woodward) remarked that the subject 

 of sandstone-dykes had not been brought before the Society, except 

 incidentally, for more than 60 years — when Strickland called 

 attention to the remarkable dykes of calcareous grit in Cromarty. 

 It was difficult to say whether those particular dykes were filled from 

 above, or by hydrostatic pressure from below, as they were seen 

 only in plan and not in section. In some cases wind-drifted sand 

 might have filled fissures. 



Prof. Jtjdd referred to the case in Cromarty which was supposed 

 by Murchison to be a ' trap-dyke ', but was afterwards shown by 

 Hugh Miller to be composed of sandstone and actually to 

 contain fossils. He suggested that the fissure might have been 

 formed by earth-movements or solution, subsequently to the 

 deposition of the Keuper Sandstone, but before its consolidation. 

 As the fissure was opened, the sand from above might gradually 

 find its way downward, and would at last be converted by soluble 

 silica, traversing the mass, into quartzite. 



Mr. Strahax remarked that he had described veins of quartzite 

 in the limestone of Flintshire. 1 In the Talargoch Mine some of the 

 veins contained an impalpably-fine siliceous sand, which passed in 

 its unweathered state into a quartzite resembling that described by 

 the Author. Such deposits tended to fill any fissure or cavity 

 in the limestone, and might be derived from any overlying sand- 

 stone, whether a bed interstratified with the limestone or, as in 

 the case referred to, from the chert-beds of the Millstone-Grit. At 

 Talargoch there was no Keuper Sandstone overlying the limestone, 

 and he was not satisfied that the material described by the Author 

 had been derived from that formation. 



The Rev. H. H. AYixwood said that he was much interested in 

 the description of the ' dykes *, a formation with which he was very 

 familiar in the Mendip district, where the joints in the Mountain- 

 Limestone were filled up by Liassic and Triassic deposits : these, 



1 ' Geology of Rhyl, &c.' Mem. Geol. Surv. (1885) pp. 47-48. 



