110 Major-Gen. McMahon — Culm-measures at Bude, Cornwall. 



Quartz. — Quartz is the most abundant material. It is in sub- 

 angular to angular grains ; these grains are rarely rounded and 

 are never water-worn. They vary greatly in size in the same 

 slice, ranging in the coarser-grained beds from one sixty-sixth to 

 one eight-hundredth of an inch in their longer diameters. The 

 orientation of such of the grains as are longer in one direction than 

 in other directions is related in two slices only to the direction of 

 the lamination ; but in these two slices comparatively few of the 

 grains are elongated and they are splintery fragments whose shapes 

 are not due to pressure in situ. With these exceptions the grains 

 of quartz, in the slices examined, are oriented in all directions. 

 The silt, of which the rocks are made up, was evidently deposited in 

 still water, and no secondary structure has been imposed upon these 

 rocks since their consolidation. 



Very few grains exhibit strain shadows, and as the grains that 

 do so are exceptionally few in number, this structure was, it may be 

 presumed, set up in them before they were deposited in the Culm 

 beds. The elongated grains do not exhibit this structure. 



Liquid, and other inclusions, in the quartz, where they take the 

 form of lines, are not oriented in any common direction ; each grain 

 has its own system, which is not related in any way to that of its 

 neighbours. 



Liquid cavities with bubbles are a common feature in the quartz 

 grains. 



Felspar. — Occasionally, but rarely, the grains of felspar contained 

 in the slices exhibit the polysynthetic twinning of the triclinic 

 system. In the majority of cases the felspar is much decomposed 

 and kaolinized. 



Mica. — A silvery mica is present in all the slices, and is fairly 

 abundant in most of them. As the important question to be 

 determined, with reference to this mineral, is whether it is an 

 original constituent of the rock, or a secondary product formed after 

 the Culm beds were deposited, I have been at considerable pains 

 to try and ascertain its species. Unfortunately the mica occurs in 

 leaves of such microscopic minuteness that it is extremely difficult 

 to isolate any of it, and impossible, I think, to obtain a sufficient 

 quantity for a quantitative chemical analysis. 



The mica is untouched by boiling hydrochloric or sulphuric 

 acids ; and after subjecting a portion of one of my specimens, 

 reduced to fine powder in an agate mortar, to prolonged digestion in 

 these acids, which apparently removed everything but the quartz, 

 mica, and carbon, I tested the residue for potash and soda and 

 found both these alkalies present. As this experiment was vitiated 

 by the possibility that though I could discover no felspar under 

 the microscope in the residue left by the acids, felspathic material 

 might nevertheless have been left behind, I set to work again, and 

 eventually succeeded in isolating a few very minute leaves of mica. 

 Having previously tested these leaves under the microscope, to make 

 sure that I had got hold of the real thing, I dissolved them in hot 

 hydrofluoric acid, and tested, by microchemical processes, with 



