Major- Gen. McMahon — Culm-measures at Bude, Cornwall. 113 



imbedded in an indurated shaly clay, bluish and slightly micaceous. 

 It is evident from the regularity with which these remains are 

 disposed — the leaves being in no case crumpled, or distorted, as well 

 as from the fine texture of the rock in which they occur — that they 

 have been deposited from water either perfectly still, or only slightly 

 in motion." 



I have several good samples of the Sub-Himalayan leaf-beds 

 above alluded to, and have studied slices of them under the micro- 

 scope. In point of structure, and in the character and arrange- 

 ment of their component grains, and leaves of mica, they are 

 indistinguishable from many of the Bude beds. If the one is 

 composed of original materials deposited in still water, this state- 

 ment must be equally true of the other. 



This conclusion regarding the Bude rocks arrived at on purely 

 microscopical evidence appears to agree well with the state of things 

 described by Mr. A. J. Jukes-Browne, F.G.S., in his Building of 

 the British Isles. After quoting from Prof. Green an account of 

 the process of sedimentation that probably went on during the 

 deposition of the Millstone Grit and Coal-measures, which is too 

 long to give here, he adds in a footnote : " It would be better 

 described as an immense delta or fenland, including many large 

 lagoons and wide channels, surrounded by swamps which were 

 never much above the level of the sea." l 



So much for their manner of deposition. As regards their origin, 

 the microscopical evidence favours the view that the materials of 

 which the Bude rocks are composed were derived from the waste 

 of a crystalline area, for they are made up of fragments of quartz, 

 felspar, and mica, with some schorl and zircon crystals — all charac- 

 teristic of granitoid rocks —together with some crystalline fragments 

 of more schistose character. It is also material to note that they do 

 not contain a single grain of such rocks as slate, or limestone. 



I see no reason to suppose that the materials were derived from 

 older sedimentary rocks, or that they travelled any great distance 

 before they found their present resting-place. The absence of water- 

 worn granules and the want of variety in the assortment of minerals 

 are opposed to both these suppositions. The sand of rivers fed by 

 extended catchment areas, or that flow for long distances over 

 several geological systems, is usually as rich in specimens as a well- 

 stocked mineralogical museum. 



The specific gravity of slate and limestone does not differ very 

 greatly from that of quartz and felspar, as will be seen from the 

 following table : — 



Clay-State (B. Von Cotta) ... Sp. G. 2-5 — 2-8 



Limestone {ib.) ,, 2-6 — 2-8 



Quartz (J. D. Dana) ,, 2-4 — 2-8 



Felspar (ib.) 2'56- — 2-75 



If then the catchment area that supplied the Culm-beds at Bude 

 contained exposures of slate or limestone, I think it is as certain as 

 anything of the kind can be, that the Bude beds would have con- 

 1 The Building of the British Isles, p. 90, footnote. 



DECADE III. — VOL. VII. NO. III. 8 



