1888.] Dr A. Geikie on the History of Volcanic Action. 345' 
Since that time he has devoted every available interval to the pro- 
secution of this research, and he now offers the completed results to 
the Society, which encouraged him by printing his earliest com- 
munication on the subject. 
I. The first part of the paper treats of the basic dykes, which 
in such enormous numbers run across the north of England, the 
north of Ireland, and the south and west of Scotland. Keasons are 
given for referring this system of dykes to the Tertiary period, as 
was first proposed by the author many years ago. He distinguishes 
two types of protrusion — (1) single or solitary dykes of great 
length, breadth, and rectilinearity, and generally less basic in com- 
position ; (2) gregarious dykes, crowded together sometimes in 
extraordinary abundance, and marked by comparative shortness, 
narrowness, irregularity of trend, and a more basic composition. 
The variations in petrographical characters are described, and it is 
shown that the material of the dykes includes rocks of basaltic, 
doleritic, and andesitic types. The geological structure of the 
dykes is then considered, especially their varying hade, breadth, 
and length, their interruption of lateral continuity, and the persist- 
ence of their mineral characters. Instances are cited of their fre- 
quent upward termination, and measurements are given of their 
vertical extension, as in the case of the Cleveland dyke, which is 
known to have come through a thickness of at least 17,000 feet of 
different strata; and in those of the dykes crossing mountain crests in 
the west of Scotland, where a difference of level of more than 3000 feet 
can be observed between different adjacent parts of the surface of the 
same dyke. Allusion is made to the occasional branching of dykes, 
to their connection with intrusive sheets, to their intersection, to 
their repeated uprise in the same line of fissure, and to the contact 
metamorphism associated with them. A discussion follows of the 
relation of the dykes to the rest of the geological structure of the 
regions traversed by them. Their total independence of that struc- 
ture, and their general parallelism, indicate terrestrial conditions 
like those postulated by Hopkins in his great memoir on physical 
geology published in 1835. The terrestrial crust over the dyke 
region, subjected to great longitudinal tension by some uplifting force 
from below, was simultaneously and rapidly rent open by thousands 
of parallel fissures having a general north-westerly direction. Into 
