TRANSACTIONS OF THE SECTIONS. 91 
why these contortions of the strata should be accumulated on particular lines, and 
not generally distributed. They occur where the crust of the earth happens to have 
been weakest; and thus the whole effects of the expansion of 1000 miles of strata 
may be found within the compass of 100 or of 10 miles. If, then, we regard moun- 
tain chains generally as the convoluted and upturned edges of strata produced by 
enormous lateral_pressure, I venture to think we shall approach much nearer to a true 
conception of the causes which have been at work, than in attributing such elevatory 
movement to perfectly vague and arbitrary volcanic forces. Another and a distinct 
cause has also been assigned for lateral pressure, which should throw the crust of the 
earth into wave-like elevations and depressions; viz. the contraction of the internal 
nucleus by cooling. The effects of the two causes would be similar, but the former 
would appear, I think, to be the more energetic and important. 
I think the application of the principle, that the elevatory forces which raise moun- 
tain chains are to be looked for in Jateral thrusts, will assist in forming a true con- 
ception of the forces which have dislocated the Lake District. Mr. Hopkins, our 
present President, has shown how the conformation of this district, the tendency in 
the valleys to converge towards a central point, may be in great part accounted for 
by the supposition that an elevatory force has acted on a line running nearly east 
and west from Shap Fells, through Helvellyn and Scaw Fell. The result would be 
a series of cracks or faults in the directions in which the valleys now run. This 
supposition I think accounts for a part of the phenomena, but not for the whole 
of them. A simple vertical force acting on the line supposed would neither account, 
I think, for the contortions of the strata in Shap Fells showing Jateral pressure, for 
instance, nor for the fact that in all the valleys on the south of the line, the eastern 
side of the fault should be elevated, the western depressed. But if we observe that 
Shap Fells, the point where this line of elevation commences, is in the corner where 
the two great lines of the Pennine and Cross Fell faults meet at an angle, and sup- 
pose the elevatory force along this line to be the resultant of two lateral thrusts 
at right angles to these two great faults, I think we shall approach a true solution 
of the phenomena observed: 
I cannot on the present occasion do more than dip into this most interesting and 
important department of Dynamic Geology; but 1 may perhaps have said enough 
to give a consistent meaning to my description of the geology of this portion of the 
Lake District. 
It only remains for me to give some reasons for the supposition that the beds of 
porphyry interstratified in the greenstone slate series have been originally aqueous 
deposits changed into porphyry by metamorphic heat, combined with pressure, and 
the presence of water. It has been remarked long ago by Sedgwick, that the slaty 
and porphyritic beds of the greenstone slate formation are not separated or distin- 
guishable from each other by any constant or clear characteristics. They run into 
each other almost indefinitely, both in passing across the strata from one bed to the 
next, and even in following the same bed along its line of bearing. This is some- 
times shown in a striking manner in the weathered surfaces of the rocks. These sur- 
faces may show externally all the marks of a stratified bed, the bedding, the joints, 
the cleavage, marked even by light-coloured stripes, and yet the first stroke of the 
hammer dispels the illusion; the rock is found to be a compact porphyry, which will 
not split according to the seeming external lines either of cleavage or joints. If any 
rocks are metamorphic, then these porphyritic beds have, I think, every external mark 
of being so. 
I may here quote the words used by Sedgwick in his last work on Paleozoic 
Rocks. Speaking of the Cambrian greenstone slate formation, he says, “The mo- 
difications of the slate are first described, and it is shown that they pass on one 
hand into compact felspathic slate, sometimes porphyritic; on the other into coarse 
granular and concretionary slaty masses, and through them into breccias or pseudo- 
breccias ; all these changes being effected without any change of strike or dip. In 
like manner it is shown that the amorphous, and even semicolumnar prismatic por- 
phyries, are not only arranged in directions parallel to the tabular masses of green 
roofing-slate, but pass themselves into a slaty texture with a strike and dip parallel 
to those of the true roofing-slates. From these facts, as well as from the negative 
facts, that the porphyries. never penetrate the raofing-slates in the form of dykes, and 
