606 REPORT— 1902. 



recop;nisable as the very characteristic Basement Carboniferous conglomerate of 

 the Cross Fell Range. 



The angidar blocks of quartzite can be matched precisely by the rocks wliich 

 succeed the Basement conglomerate of Roman Fell. The author at one time 

 regarded the rhyolites as indisputable evidence of the exposure of the Borrowdale 

 rocks of the Cross Fell iulier, and denudation during Permian times ; but, while this 

 still seems to be the most probable explanation of their presence in the Upper 

 Brockram, it is possible that they could have been derived from the Carboniferous 

 Basement conglomerate, in which at Swindale Beck a few such pebbles occur. 



Setting aside the rhyolite pebbles, there is still a body of evidence which 

 seems to warrant deductions of very great interest. The facts to be explained 

 are the occurrence in the Lower Brockram of a practically pure gathering of 

 Carboniferous limestone, while the Upper Brockram contains a very high percen- 

 tage of rocks from the very base of the Carboniferous series. They mi?ht be explained 

 on the supposition of derivation from opposite sides of the Vale of Eden, the Lower 

 Brockram being supposed to come from the Carboniferous limestone outcrop 

 towards Orton, while the Upper Brockram was derived from the Pennine Range. 

 This view has little to commend it. If the Carboniferous basement conglomerate 

 were exposed to denudation during the deposition of the Upper Brockram, then 

 the Carboniferous limestone must have formed a bold escarpment at the same time ; 

 and, that being granted, it is highly improbable that it failed to yield the materials 

 of the Lower Brockram, which at Tlungriggs is less than three miles from the 

 outer Pennine Fault which exposed a series of Carboniferous rocks in Permian 

 times. Upon the alternative and, as it seems, preferable hypothesis that the 

 materials of the two Brockrams were all derived from the Pennine Chain, an inter- 

 Permian movement of the faults which throw up the Cross Fell Range and the 

 well-known inlier seems necessary. 



Professor Lapworth has pointed out that when an anticlinal fold is exposed to 

 denudation the derivative beds will consist of the same materials as those of the 

 anticline, but in reverse order ; the uppermost beds of the anticline will yield 

 pebbles of the lowest of the derivative beds, while the core of the anticline will be 

 represented only in the highest of the derivative beds. 



This principle may be illustrated by the Tertiary beds of the south-east of 

 England. The Lower Eocene conglomerates contain only flint pebbles from the 

 chalk, while the high-level gravels which rest on the Bagshot series contain, 

 besides flint, many pebbles derived from the Lower Greensand. 



Where, however, the exposure is by a fault scarp, the whole of the beds 

 exposed ia the scarp will contribute to the first formed derivative conglomerates. 

 The absence of detritus of the basement beds of the Carboniferous from the Lower 

 Brockram shows that the basement beds were not exposed in early Permian times, 

 but a movement of the fault exceeding the thickness of the Penrith Sandstone 

 brought the lowest members of the Carboniferous series above the surface at the 

 time of the deposition of the Upper Brockram. 



9. Report on the Erratic Blocks of the British Isles. — See Reports, p. 252. 



10, The Conditions under which Manganese Dioxide has been Deposited 

 in Sedivientary Rocks, as illustrated by the Elgin Sandstones. By 

 William Mackie, M.A., M.D. 



Manganese dioxide has been observed to occur in the Elgin sandstones under 

 the following conditions : — (1) In ovoid or rounded spots, from ^ inch to 6 inches in 

 diameter, known to the quarrymen as ' vegetations,' at Newton Quarry, in U.O.R. 

 rocks. From analyses MnO.j varies from -180 per cent, to '262 per cent. (2) In 

 small nodules, about \ inch in diameter, in Triassic rocks, S.E. of Cuttishillock, 

 MnO„ = 12 87 per cent. (3) In punctiform spots around decomposing felspars in 



