694 J. W. JUDD ON THE SECONDARY ROCKS OE SCOTLAND. 



conformably by the Infralias limestones; but at many points its 

 members are seen to be overlapped by the Cretaceous beds. 



It is very characteristic of the Poikilitic beds to vary wonderfully 

 in thickness and character within short distances. Nowhere is this 

 more strikingly illustrated than in the two opposite sides of the 

 Sound of Mull. On the Morvern side of the Sound, at the Innimore 

 of Ardtornish, the series is some 400 or 500 feet thick; but on the 

 Mull side, at Craignure (Auchnacroish), it is found to be reduced to an 

 insignificant amount. The strata at this latter locality are greatly 

 metamorphosed, the sandstone pebbles being converted into quartzite 

 and the limestone into marble, and the colours of both frequently 

 discharged. The igneous intrusions are so numerous and compli- 

 cated that the exact thickness and succession of beds in it is as 

 difficult to estimate as in the analogous case of Ardnamurchan. At 

 the very base of the series, and lying directly on the older rocks, is 

 a breccia composed of angular fragments of gneiss and quartzite; but 

 this can only be seen at low water during spring-tides. 



In Duart Bay, and at some other points along the west side of the 

 Sound of Mull, we find more or less distinct traces of the beds of the 

 Poikilitic series. In some of these cases the exposures are so small, 

 and in others the beds have undergone such an amount of metamor- 

 phism, that it would be idle to attempt to do more than chronicle 

 their existence. In some cases, as opposite to Calla Island and below 

 the precipitous cliffs of Toum Peroch, there are strata which, in 

 their isolated position and their altered condition, it is impossible to 

 state absolutely to belong to the Poikilitic, though perhaps the balance 

 of evidence is in favour of such a determination. The consideration 

 of the age of these doubtful deposits need not, however, detain us 

 further. 



The last point towards the south where the Poikilitic strata are 

 seen is in the small island of Inch Kenneth, on the west side of 

 Mull, and on the shores of the peninsula of Gribun, opposite to it. 

 Here the unconformity between the Poikilitic strata and the older 

 rocks, which here consist of purplish quartzite and gneiss, is quite as 

 beautifully exhibited as at Gruinard Bay ; and the following sketch 

 taken near the little cave known as Beg's Cave will afford a good idea 

 of the relations of the strata at this point (see fig. 8). 



On the eroded edges of the old metamorphic rocks, which lie in 

 vertical and contorted positions, lies a very coarse conglomerate, 

 often containing angular and subangular fragments, a breccia com- 

 posed of such fragments being always found at the base. Tracing the 

 conglomerates upwards we find them alternating with and passing 

 into irregular bands of greenish, and mottled sandstone, with some 

 beds of concretionary limestone and occasional marly beds and thin 

 layers of gypsum. 



The thickness of the Poikilitic strata at Gribun is considerable ; 

 and their summit is not seen, as they are unconformably overlapped 

 by the beds of the Cretaceous series. Indeed, as we shall see here- 

 after, the beds of the Upper Greensand are so similar in character, 

 being made up of materials derived from the Poikilitic, that it is 



