CHAP. LI 
SUM MAR 
Y AND GENERAL DEDUCTIONS 
469 
variations in the upper limit of dykes and inequalities 111 the configuration 
of the overlying ground, suggest that where the subterranean magma has 
ascended to within a comparatively short distance from the surface a 
difference of a few hundreds or thousands of feet 111 the depth of overlying 
rock, such as the difference of height between the bottom of a valley and t 
tops of the adjacent hills, may determine the path of escape foi the n g 
through the least thickness of overarching roof. . , , 
5 Volcanic phenomena cannot be regarded as a mere isolated and 
incidental feature in the physics of the globe. During the short time within 
which man has been observing the operations of existing volcanoes he lias 
hardly yet had sufficient opportunity of watching how far they can be 
correlated with other terrestrial movements. Nor when he ende^our 
trace some such connection among the records ot the geological past, has 
yet collected materials enough to furnish a sufficiently broad and him basis 
of comparison. One formidable obstacle is presented by the difficulty m 
determining chronological equivalents in separated groups ot rock Oeolo is 
have tried to discover whether the volcanoes of some particular period or 
region were in any way connected with such geological changes as extensive 
plication, dislocations of the crust, or elevation of mountain-chains. In 
regard to the volcanic history of Britain, various possible relations of this 
kind obviously suggest themselves. Thus the division of geological time 
comprised within the Lower Silurian period was undoubtedly an interval of 
considerable terrestrial disturbance in western Europe. Ihe unconformabili- 
ties and overlaps in the series of formations belonging to that period, the 
frequent conglomerates, the great and often rapid changes m the thickness 
and lithological characters of the strata, all point to instability of land-sui 
and sea-floor. During these oscillations a prolonged and widespread series 
of volcanic eruptions took place. The volcanic manifestations | began ^ m 
Cambrian time and continued in intermittent activity till towards the 
of the deposition of the Lower Silurian formations. It is certainly a 
significant fact that the Upper Silurian deposits, in their hthologiCc 
characters, present a strong contrast to those that preceded turn • T 
point, on the whole, to quiet sedimentation, during an interval of compaiat 
calm in the terrestrial crust. With this evidence of tranqmlhty there ^ 
over almost the whole of the British Isles, an entire absence of an ^ trace 
renewed volcanic activity. With the exception ol 
in the extreme west of Ireland, not a single undoubted instance yet 
of an Upper Silurian volcano. . , f . 
After the deposition of the Upper Silurian rocks an interval of great 
terrestrial disturbance ensued, and these rocks over a large part of Bntam 
W ere intensely plicated and crushed. The movements, continued into the 
neriod of the Lower Old Bed Sandstone, were, m their later stages, acc 
panied or, at least, followed by the vast outpourings of lava which now cover 
s 0 much of the tracts of Old Bed Sandstone 111 Scotland and Ireland. 
In proportion as the volcanic energy was vigorous, widespread and 01 0 - 
1 Trans. Geol. Soc. Edin. vol. ii. part m. (1874). 
