86 -PROCEEDIJSTGS OF THE GEOLOGICAL SOCIETY. 



took place between some of the eruptions. Eoiind the vents, indeed, 

 where the successive sheets of volcanic material follow each other 

 continuously, it is perhaps impossible to form any definite opinion 

 as to the relative chronological value of the lines of separation 

 between different ejections. But where some hundreds of feet of 

 coarse conglomerate, chiefly composed of well-rounded porphyrite 

 blocks, intervene between two streams of porphyrite, we may 

 conclude that the interval between the outpouring of these lavas 

 must have been of considerable duration. Other evidence of a 

 similar tendency may be recognized in the intercalation of groups 

 of varied sedimentary accumulations, such as those which were 

 deposited over the site of Eastern Forfarshire and Kincardineshire 

 during the time that elapsed between two successive floods of lava. 

 In the Den of Canterland, for example, in the midst of the volcanic 

 sheets we find interesting evidence of one of these intervals of 

 quiescence, during which layers of fine olive shales were laid quietly 

 down, while macerated vegetation, drifting over the lake-bottom, 

 was buried with egg-packets of Pterygotus, remains of fishes, and 

 abundant gally-worms {Campecaris) washed from the neighbouring 

 land. So undisturbed were the conditions of deposition that cal- 

 careous sediment gathered round some of the organisms and encased 

 them in limestone nodules. 



2. Vents. — I will now refer to some of the vents from which this 

 vast mass of volcanic material was ejected, and to the structure of 

 the rocks that have filled them up. It must be borne in mind that 

 across the centre and south of Scotland a number of bosses of 

 igneous rock occur which may plausibly be referred to the volcanic 

 phenomena of the Lower Old Red Sandstone, but cannot be proved 

 to be actually part of them. I allude more particularly to the 

 bosses of granite and other acid rocks which rise through the 

 Silurian strata of the Southern Uplands.^ The largest are the well- 

 known masses of Galloway, with which must be grouped the bosses 

 near New Cumnock, that of the Spango Water, and those of Cock- 

 burn Law and Priestlaw in Lammermuir, together with a number 

 of masses of felsitie material scattered over the same region, such 

 as the Dirrington Laws of Berwickshire. These bosses present 

 some points of structure in common with true vents. They come 

 like great- vertical columns through highly-folded and puckered 

 strata, and'^ as they truncate Llandovery and Wenlock formations, 



^ I suggested this possible connexion many years ago in Trans. Geol. Soc. 

 Edin. Tol. ii. (1874)p. 21. 



