&8G J. W. JTJDD ON THE SECONDARY ROCKS OF SCOTLAND. 



tween the Innimore of Ardtornish and the opposite shore of Mull ; 

 and that it is unconformably superposed on these old rocks there can 

 be little room for doubting. We have already mentioned that the 

 Poikilitic beds of the reefs to the west and in the cliff-section of the 

 Ardtornish ravine clearly lie above the Carboniferous strata ; but as 

 to the question whether an unconformity exists between these two 

 series, I have altogether failed in obtaining satisfactory and conclu- 

 sive evidence, so imperfect are the exposures at this locality. 



The wonderful interest attaching to these Carboniferous strata — 

 forming a patch so minute in dimensions and so isolated in position 

 — arises from the fact that they afford evidence of the extension of 

 strata of the Carboniferous age into areas in which they were formerly 

 unknown and were believed never to have been deposited. Sir 

 Roderick Murchison and other authors have argued that the absence 

 of Carboniferous rocks in the Scottish Highlands must be accepted 

 as proof that such strata were never deposited in the area, and that 

 the rocks now forming that mountain district constituted at a period 

 so early as the Carboniferous a continuous land-surface. Eut here, 

 as in so many other cases in the same district, we find a most striking 

 illustration of the effects produced by denuding agents during past 

 geological epochs, and a warning as to the necessity for exercising 

 the greatest caution in drawing deductions from the absence of strata 

 in a given area as to whether they were ever deposited in it. 



E. The Poikilitic System. 



To the indefatigable and sagacious Macculloch we are indebted 

 for first pointing out that there exists in the Western Highlands at 

 Gruinard Bay evidence of a series of Red Sandstone strata uncon- 

 formably overlying, and therefore of far later date than, the great 

 masses of Red Sandstone (Torridon Sandstones) which constitute a 

 grand and widely spread formation in the north-western part of the 

 British archipelago. Macculloch, Murchison, Sedgwick, and Nicol, 

 assuming (as was universally done by the earliest students of High- 

 land geology) that the Torridon Sandstone represented the " Old 

 Red," saw that the most probable position to assign to the unconform- 

 able strata would be that of the " New Red." Prof. Nicol has added 

 many new and exact details concerning these interesting beds in a 

 paper read before this Society in 1857*. 



But it is evident that the admission, now very generally made, 

 that the Torridon Sandstones are 'of older date than the Old Red, 

 once more places the question of the age of the overlying deposits 

 in a state of uncertainty. 



Other deposits of similar character to the strata of Gruinard Bay 

 have been referred to as existing in Raasay, and by Dr. Bryce were 

 thought to be possibly of Rhaetic age. 



These and a great number of scattered masses of conglomerate, 

 sandstone, marl, and concretionary limestone I have now been able 



* Quart. Journ. Geol. Soc. vol. xiv. p. 167. 



