C. LAPW0RTE ON THE MOFFAT SERIES. 251 



In this section consequently the first grey-shale group attains a 

 collective thickness of about 77 feet. If we connect each of its non- 

 fossiliferous bands with the special Graptolitiferous seams immediately 

 above, the whole group may be regarded as falling into three distinct 

 zones. The first of these zones is marked by the exclusive presence 

 of Rastrites maximus, the second by that of Monograptus spinigerus 

 and the third by the preponderance of Diplograptus cometa. 



Next in importance to the very distinctive aspect given to the 

 group as a whole by the alternation of broad bands of grey and 

 black shale, the most striking mineralogical feature is formed by its 

 intercalated seams of white clay or mudstone. These characteristic 

 beds vary in thickness from an inch to a foot, and, when weathered, 

 break up into small flakes about the size and thickness of a finger- 

 nail. These clay bands abound in the South Cliff and in the walls 

 of the corrie ; their thin chaff-like fragments are scattered over the 

 whole section of the grey-shale group, giving it a most peculiar 

 appearance, and effectually distinguishing it from all the remaining 

 beds exposed in the lateral gorge. 



(ii.) First Black-Shale Group. 



Continuing our catalogue of the rocks of the glen, we notice that 

 the grey-shale group is followed by a thick series of black shales, 

 crowded with fossils, and arranged in the following order : — 



(a) A mass of black flaggy shales and mudstones 30 feet in 

 thickness. In the first half of this zone the beds are thin, soft, and 

 highly pyritous ; the remainder are hard and flag-like. Throughout 

 the whole mass there occur at intervals seams of blue, yellow, or 

 orange-coloured mudstone or clay, varying from a few inches to 

 nearly a foot in thickness. Many of these brightly coloured beds 

 contain lines of calcareous concretions and balls of nodular iron- 

 stone. One very large band of nodules occurs in the very centre of 

 the zone, dividing it into two portions, lithologically and palaeontologi- 

 cally distinct. 



The characteristic fossil of the whole zone is Monograptus gre- 

 garius (Lapw.). Its first division is marked by the exclusive presence 

 of Rastrites peregrinus, and the second by that of M. Sandersoni. 



These beds, after a slight contortion in the walls of the corrie, are 

 cut off abruptly from the strata which naturally follow them by a 

 north and south fault, crossing the line of strike of the rocks at a 

 small angle, and bringing them against a great thickness of pale 

 unfossiliferous mudstones to be described in the sequel. 



Leaving the lateral gorge at this point, and examining next the 

 beautiful section in the Main Cliff (Sect. II. Plate XII.), we notice 

 that the grey-shale group of the corrie crosses the cliff immediately 

 below its highest point. It is in such a position that it cannot be 

 reached with safety, but the grey, black, and white bands are easily 

 identified from below. Instead of lying upon the coarse grits of the 

 falls, as in the lateral gorge, it here visibly passes underneath them 

 at an angle of about 45°. 



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