72 :rkoceedings of the geological society. 



and diabase lavas, like those of Central Scotland, aiford evidence of 

 contemporaneous volcanic action in the wide tract of Lower Old Eed 

 Sandstone which stretches from Pomeroy to Enniskillen. 



In spite, therefore,, of the effects of powerful faults and of the 

 unconformable overspread of younger formations, the position of at 

 least eight separate groups of volcanoes can be more or less definitely 

 fixed along the site of the great basin in which the Lower Old 

 Eed Sandstone of Central Scotland and the N'orth of Ireland was 

 deposited. 



In the following sketch of the chief characteristics of this volcanic 

 region, I shall refer (1) to the petrography of the various groups of 

 rock ; (2) to the manner in which they are arranged in the general 

 geological structure of the ground; taking in consecutive order 

 (a) the lavas and tuffs of the superficial outpourings, (b) the vents 

 with the materials now filling them, and (c) the dykes and sills, 

 some of which may be regarded as records of the closing phases of 

 the volcanic activity of the period, 



II. Petrogeaphy. — In presenting a general summary of the petro- 

 graphical characters of the igneous rocks of the Lower Old Eed 

 Sandstone, it may be convenient to include not only those of ' Lake 

 Caledonia,' but also some which have been obtained from other 

 basins, though, on the whole, the prevailing types in one region are 

 found to be repeated in the others. 



1. Bedded Lavas. — Beginning with the lavas which were poured 

 out at the surface, we have to notice a considerable range of chemical 

 composition among them, although as a rule they are characterized 

 by general similarity of external appearance. At the one end come 

 diabases and other ancient forms of basalt or dolerite, wherein the 

 silica percentage is below or little above 50. By far the largest pro- 

 portion of the lavas, however, are porphyrites (altered andesites), 

 having about 60 per cent, of silica. With these are associated lavas 

 containing more or less unstriped felspar and a somewhat higher 

 proportion of silica, which may be grouped as trachytes, though no 

 very sharp line can be drawn between them and the andesites. It 

 is possible that some felsitic or rhyolitic bands noticeable in the 

 Pentland Hills and elsewhere may have flowed out as acid lavas. 



It is noteworthy that the lava-sheets of the Lower Old Eed Sand- 

 stone, if we consider the character of the prevalent type, hold an 

 intermediate grade between the average chemical composition of 

 those of Silurian and of those of later Carboniferous time. On the 

 one hand, they hardly ever assume the character of thoroughly acid 



