XXXIV INTRODUCTION. 
After the summer of 1822, the exposition of cleavage-planes (as given above) formed 
a part of my public teaching, and was a matter of frequent discussion. And so far from 
overlooking the fact that, under proper limitations, the strike of the cleavage-planes coincided 
nearly with the strike of the beds, I absolutely assumed, in my traverses among the higher 
Cumbrian mountains in 1823 and 1824, that the two strikes were so nearly coincident that 
the strike of cleavage (which was in numberless instances visible when the true bedding was 
concealed) might be assumed as a good approximation to the strike of the beds. But 
several years afterwards, though the above rule might still be considered approximately 
true, I found so many limitations to it, that I was compelled to notice them whenever 
I described the conditions of the cleavage-planes of slate-rocks, especially in Devonshire, 
Cornwall, and a portion of South Wales*. 
When my paper on the cleavage-planes of slate-rocks and of large mineral masses was 
first read, it was attacked by one of the most distinguished members of the Geological Society 
in terms of extreme vituperation, and its statements were for some time controverted 
by the Director of the Government Survey. And after the paper was published, it was 
attacked, by a third gentleman, in one of the Philosophical Journals, in terms I might almost 
say of contemptuous bitterness. But, all theory apart, it was a true statement of very 
important facts of structure, and it gradually was accepted as true; and though not perfectly 
original, it has been the means of helping subsequent observers in making their way among 
some of the most perplexing phenomena of geology. 
That the cleavage-planes, in a mountain-chain of slate-rocks, have been produced by the 
compounded effect of all the crystalline forces acting on the mass, receives a remarkable 
confirmation from certain phenomena exhibited by the protruding bosses of granite in the 
south-western parts of Cornwall. The granite is a mineral aggregate of a very common 
kind; and its crystals were formed by the mutual action of its molecules at the time it 
was passing into a solid state. It gives no appearance of a slaty structure; yet it cleaves 
(sometimes through large spaces) in one constant direction, and in nearly parallel planes, 
much more readily than in any other. And this direction is known to the quarrymen by 
the approximate arrangement of some of the larger crystals of felspar, upon the same 
planes+. The phenomena prove, I think, to demonstration, that while the crystalline 
* For example: I made many additional observations on the modifications of cleayage-planes in Cornwall in 1828; 
in Wales in 1831 and 1832; and in Devonshire and Cornwall in 1836, 1837, and 1838. The statement of the text may 
perhaps seem out of place in this Introduction. I make it however here, because my views have been misrepresented. 
Thus, in an Anniversary Address ex cathedrd, the President of the Geological Society of London asserted that I had over- 
looked one leading fact, viz. that the strike of the cleavage-planes was nearly coincident with the strike of the beds: and 
the same assertion was virtually repeated in a printed memoir by the only member of the Society who has written, at any 
length, on the theory of cleayage-planes. Neither of these gentlemen (at the time their statements were made public) would 
appear to have so much as looked at my Paper on cleavage-planes. (Geol. Trans. 2nd Series, Vol. 1, pp. 473 and 483.) 
+ This structure is perfectly understood by the quarrymen. It was well described in a paper (to which I am not 
able to refer) by Mr R. Fox and Mr Eneys—two Cornish gentlemen who haye made valuable contributions to physical 
geology. It was also alluded to by myself in the paper referred to in the previous note, Geol. Trans. 2nd Series, Vol. ut. 
p. 483. 
