16 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [April 22, 



abundance ; but they are there more regular, and generally keep 

 parallel to each other in a north-west direction. 



On Pabba Island they are also numerous, running along the line 

 of the natural joints, with the usual north-westerly strike. Here 

 they show very unequal degrees of permanence, in some cases 

 crumbling away as rapidly, or even more so, than the surrounding 

 shale ; in others jutting up like walls. Of the latter kind one very 

 noticeable example occurs on the northern shore of the island. A 

 dyke, from 3 to 4 feet thick, and, next the cliff-line, about 30 feet 

 high, crosses the beach and runs out to sea (MS. Sketch No. 11). 

 The shale has been washed away on all sides of it, and it blocks 

 up the walk along the beach like a wall of smoothly built ma- 

 sonry. These Pabba dykes are of interest in so far as they enable 

 us to estimate the extent to which trap-rocks have altered the strata 

 of the district ; for in Strath there is so much metamorphism and 

 so many protrusions of syenite, that one is apt to miscalculate the 

 amount of influence of the different igneous eruptions. Pabba, how- 

 ever, is nearly two miles distant from the nearest point of syenite, so 

 that this rock could not have had any effect upon the shales of the 

 island. There is thus no conflicting agency to mar or heighten that 

 of the trap-dykes ; and, as in Pabba they fully equal in number those 

 in the most metamorphic regions of Strath, it follows that, if, either 

 wholly or in part, they have produced the metamorphism of Strath, 

 they must have caused a corresponding amount of alteration in Pabba ; 

 but in that island there is no metamorphism beyond the mere hard- 

 ening of the shales in immediate contact with the trap. At the di- 

 stance of 2 feet they are generally as soft and fissile as when furthest 

 removed from dykes. It is evident therefore that the remarkable 

 metamorphism I have yet to describe cannot have been produced by 

 the trap-dykes, startling as their number may seem. Their effect is 

 limited to a few inches from their edges, and even this slight altera- 

 tion becomes undiscernible in the interior, where it is lost in a far 

 more extended and complete metamorphism. 



The trap-dykes, like the syenites, are divisible into two well-marked 

 classes, differing from each other in mineralogical texture and in age. 

 The one group comprises nearly all the dykes of the district, and is 

 formed of a dark-grey or bluish-black basalt*, not columnar, but 

 much jointed. The rock is exceedingly hard, and weathers with a 

 greenish-brown crust. The dykes of the other class are not nume- 

 rous, and consist of a dark crystalline augitic greenstone*, which 

 sometimes approaches the basalt in texture. 



The annexed ground-plan (fig. 10) of the neighbourhood of the 

 old Manse at Kilchrist shows the relation of these two classes to 



* These terms are here used, as they have, I think, always been in Scottish mi- 

 neralogy, to signify two rocks differing from each other, not in composition, but in 

 texture ; basalt being a compact black mixture of augite and felspar without visi- 

 ble crystals, greenstone a lighter-coloured mixture of the same minerals, the cry- 

 stals being easily recognizable. When hornblende replaces the augite, it is called 

 a hornblendic greenstone. In Scotland, where so large a proportion of the traps 

 are augitic, this distinction is a very useful one. 



