142 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



at a long subsequent stage in this volcanic period may well have 

 been considerabl}- more acid. 



In actual fact, as I have already remarked, the petrographical 

 range of the sills varies from picrite to dolerite without olivine, and 

 in some districts even to such rocks as felsite and quartz-porphyry. 

 But the great majority of these sheets in the basin of the Firth 

 of Porth, where they are chiefly displayed, are dolerites (diabases), 

 either with or without olivine. 



The structure of these intrusive sheets and their contact-meta- 

 morphism have been fully described, and some of them have become, 

 as it were, ' household words ' in geology.^ Exposed in so many 

 fine natural sections in the vicinity of Edinburgh, they early 

 attracted the notice of geologists, and furnished a battle-ground on 

 which many a conflict took place between the Plutonist and Nep- 

 tunist champions at the beginning of the present century. 



While one is struck with the great size and extent of the sills 

 now under consideration, as compared with the small and local 

 sheets underneath the plateaux, there is a further fact regarding 

 them that deserves remark — their capricious distribution. Their 

 occurrence seems to have little or no relation to the measure of 

 volcanic energy as manifested in superficial eruptions. 



Thus in the north of Ayrshire, where a long band of lavas and 

 tuff's, pointing to vigorous activity, lies at the top of the Carboni- 

 ferous Limestone series, and where the strata underneath it are 

 abundantly exposed at the surface, the sills occur as thin and 

 inconstant bauds in the central and eastern parts of the district 

 only. The bedded lavas and tuff's at the head of the Slitrig Water 

 have no visible accompaniment of sills. In the Limerick basin, 

 also, no clearly separable sills have been recognized, though possibly 

 a more rigid scrutiny may yet discriminate them among the lower 

 parts of the bedded lavas ; but they have not been found among 

 the tufi^s below these lavas, nor in the limestones which underlie 

 them. Again, in the Edinburgh and Burntisland districts, the siUs 

 bear a much larger proportion to the amount of bedded lavas and 

 tuffs than they do in the Bathgate and Linlithgow district, where 



1 See, for instance, Maelaren's ' Geology of Fife and the Lothians,' 1889 ; 

 ' Geological Survey Memoir on the Geology of Edinburgh' (Sheet 32), 1861 ; 

 Trans. Eoy. Soc. Edin. vol. xxix. (1879) p. 437; Allport, Quart. Journ. Geol. 

 Soc. vol. XXX. (1874) p. 553 ; Teall, ' British Petrography,' p. 187 ; E. Stecher, 

 ' Contacterscheinungen an schottischen Olivindiabasen,' Tschermak's Mineralog. 

 Mittheil. vol. ix. (1887) p. 145 ; Proc. Eoy. Soc. Edin. vol. xv. (1888), p. 160. 



