122 PROCEEDINGS OP THE GEOLOGICAL SOCIETY. 



and tliat of Eenfrewshire just referred to is such as might have 

 been produced by the long-continued ascent of steam and heated 

 vapours through fragmentary materials left in the funnel after the 

 explosions had ceased.^ 



3. Dylces and Sills, — Dykes occur in considerable numbers around 

 the vents, but are less noticeable in the heart of the plateaux at a 

 distance from these vents. Thus they have been found abundantly 

 traversing the red sandstones that underlie the volcanic series 

 through which so many vents rise between Pintry, Strathblane, and 

 Dumbarton, and between Gourock and Ardrossan. Some of the 

 larger vents are traversed by a network of dykes and veins. The 

 large vent above referred to among the Campsie Pells is a good 

 example of this structure, and a still more remarkable illustration 

 is furnished by the great Eenfrewshire vent.^ 



The great majority of the dykes consist of felsite, resembling in 

 lithological characters the material of the necks and doubtless con- 

 nected with its uprise. There occur also dykes of andesite and 

 dolerite. Some of these, especially those which run for many 

 miles cutting every rock in the districts in which they occur and 

 crossing large faults without deviation, are certainly long posterior 

 to the Carboniferous period. Whether the small inconstant dykes 

 of more basic composition found in the same districts with the 

 felsites are to be looked upon as part of the volcanic phenomena of 

 the Plateaux is a question to which at present no definite answer 

 can be given. I shall have occasion to show that in the next vol- 

 canic period the lavas that flowed from the Puys were more basic 

 than those of the Plateaux and that they are associated with more 

 basic dykes and sills. In Roxburghshire, where it is so difficult to 

 distinguish between the denuded vents of the two periods, the dark 

 heavy olivine-basalts and dolerites of the bosses may, as I have 

 already remarked, be connected rather with the later than with the 

 earlier volcanic episode. And if that be their true age, the dykes of 

 similar material may be connected with them. 



The sills associated with the plateau-type of Carboniferous vol- 

 canic action form a less prominent feature than they do among the 

 earlier Palaeozoic formations or in the puy-type which succeeded 

 them. They consist in general of short lenticular sheets of felsite 

 like that of the necks and dykes in proximity to which they 



1 Explanation of Sheet 31, Geol. Surv. Scotland, p. 16. 

 ^ The group of veins shown in fig. 277 of my ' Text-book of Geology ' occurs 

 in this Eenfrewshire vent. 



