Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/285

 the nature and disposition of this rock. Here it is disposed in thin beds, rarely exceeding a few inches in thickness, and is inclined towards the east in an angle of about 26 degrees. These beds are divided by natural joints, at angles with the plane of stratification, and in consequence of this they break into prismatic rhomboidal or triangular fragments, which on the upper and long-exposed surfaces are so numerous and minute that the whole presents at first sight a set of sharp points and projections, among which the stratified disposition is scarcely perceived until they are more closely examined. The same disposition into thin strata is every where found throughout the larger masses, and if sometimes more obscure, it can nevertheless be always discovered by an attentive examination.

With respect to the composition of this rock it is almost always found to be a compact splintery quartz, scarcely distinguishable in small fragments from that mineral as it occupies veins. Occasionally it becomes more or less granular, and now and then will be found to contain grains of felspar, as it does in so many other places. Its prevalent colour is white, but it sometimes assumes a rusty colour at a small depth from the surface, and in some rare instances may be observed of a pink and of a brown-reddish hue.

If there is any transition between this rock and those with which it is associated, it must be sought among those beds which approach to a granular structure, between which and the most compact varieties of the red sandstone the difference is not extremely great. I cannot say that I have positively ascertained such a transition, nor am I aware that the nature of the ground is such as to admit of an examination sufficiently connected and extensive to answer this purpose. I shall however point out to geologists that part of