XXXVi INTRODUCTION. 
regular and nearly vertical ‘strike-joints,) (and partly also by\(dip-joints; They also flake off, 
here and there, into masses, more or less tabular, which are parallel to the cleavage-planes 
of the slates. It appears, therefore, that the forces which produced the cleavage-planes 
acted also, sensibly, on the very coarse conglomerates. Moreover, the conglomerates are 
separated into beds; which, however obscure, were proved to be the true beds by the organic 
remaius I collected from their planes of junction both in 1846 and in 1854. The case 
admits of no ambiguity. Now when we examine the open mechanical joints, we find that 
they have actually cut through the pebbles of quartzite and other hard masses which enter 
into the composition of the conglomerates. But when we examine the divisional planes, that 
are parallel to the cleavage-planes of the slates, the appearance is entirely different. In 
this second system of divisional planes the pebbles have not been at all affected or moved 
from their place; and they stand out in high relief on the surface of the exfoliating masses. 
What then has produced this tendency to a regular exfoliation? A new arrangement of 
the earthy, gritty, and micaceous particles which form the cement of the conglomerates. 
This cement has been re-arranged by the same crystalline action (1 know no other name 
by which it can be described) which has produced the cleavage of the neighbouring slates. 
Here then we have the clearest proof of a mechanical action like that brought to bear in 
the theory of Mr Hopkins; and in the same rock we have an equally clear proof of another 
kind of action which cannot have been produced either by a divellent force, or any con- 
ceivable mechanical force like that of pressure*. I conclude, therefore (while putting out of 
account all opinions which are theoretical), that I have, during the progress of Paleozoic 
Geology, given such a real account of the phznomena of cleavage-planes as has enabled 
other observers to make their way, with comparatively small difficulty, through some of the 
complicated sections of Wales and Cumberland. 
3. The third dificulty arises from the extraordinary development of eruptive and 
stratified igneous rocks in the central parts of the Cambrian series. When I began, in the 
* I do not accept Mr D. Sharpe’s mechanical theory of cleavage-planes, for the following reasons. (1) It is not 
based on a sufficiently wide induction of facts: it is, at best, but an ingenious hypothesis. (2) It is directly opposed to 
a multitude of positive facts, of which it gives no explanation. (3) Its author has made some great mistakes in what may 
be called the solid geometry of the subject, which in limine throw doubts upon the accuracy of his observations. (4) It gives 
no adequate explanation of the rearrangement of the molecules in cases of most perfect cleavage planes. (5) It offers no 
explanation of the disappearance and reappearance of cleayage-planes in mineral masses which have been under the same 
conditions of mechanical pressure. (6) It gives us no account of the entire absence of transverse cleavage-planes in large 
regions of old contorted Cambrian rocks; such (for example) as those in the frontier chain of Scotland. (7) Lastly 
(though this is an argument I do not wish to press against it), it seems to be out of harmony with Mr Sharpe’s theoretical 
explanation of the complicated structures exhibited by the old metamorphic rocks of the northern Highlands of Scotland. 
The frontier chain of Scotland (from St Abb’s Head to the Mull of Galloway) requires a few additional words of notice. 
It is generally made up of beds which are highly inclined and much contorted. It exhibits all the conditions of an 
elevation implying an enormous lateral pressure. It comprehends many subordinate tracts of country, where the rocks 
scem of a structure well fitted to receive the impress of a transverse slaty cleavage. It contains some good quarries of 
roofing slates, which, however, are not derived from oblique cleavage-planes; but, in every instance where I have seen them, 
are exfoliations parallel to the bedding. The theory of mechanical pressure fails to give us any good account of these 
negative facts. The crystalline theory has a ready reply to explain them: viz. the almost entire absence of calcareous 
matter, and a prevalent coarse mechanical structure unfavourable to the development of true cleayage-planes through the 
great mineral mass. 
