INTRODUCTION. XXXV 
action of the molecules produced the mineral aggregate we call granite, a more general 
crystalline action produced a peculiar and parallel arrangement of the whole mass. In a case 
like this, a merely mechanical force (like that of compression) is entirely out of question: and 
we have a full right to make use of such facts in illustration of the more obscure pheno- 
mena of cleavage-planes through slate-rocks and deposits which are mechanical. All I 
contend for is, that the phenomena of cleavage-planes are due to a crystalline rather than 
a mechanical action. 
While my first observations on cleavage-planes were made—during long by-gone years— 
in Cumberland, I had hardly noticed the phenomenon of a second cleavage-plane: but on 
many occasions I have subsequently collected, from various parts of England, a considerable 
and unpublished mass of materials, in illustration of this second plane. I should not have 
noticed it here, but from its connexion with a mechanical theory which I do not think 
even approximately true. The second cleavage-plane is generally inclined at a great angle 
to the first plane. Most beautiful examples of this double structure were seen (in 1839) 
by Sir R. I. Murchison and myself in the quarries of the Ardennes, where the fine, glossy 
surfaces of the slates are frequently marked by the parallel strizw of second cleavage; and the 
economical value of the slates is sometimes much deteriorated by the second plane. By 
a powerful reflected sun-light I have frequently been able to trace these striz of a second 
cleavage on the surface of the Bangor slates which have been brought to Cambridge. 
Instructive, though coarse, examples of this double structure are seen, here and there, in 
Devon and Cornwall; and also in North Wales, especially among the Upper Bala rocks, as 
well as among the slaty varieties of the Wenlock group. 
It has been proved by Mr Hopkins, that when a great mass of strata is pushed out of 
its place by a vertical movement of elevation, it will be acted on by two sets of divellent 
forces that are nearly at right angles to one another. Have these forces been the mecha- 
nical cause of double cleavage-planes? I reply, no: because mere mechanical forces cannot 
account for the phenomena, which imply a positive re-arrangement of the molecules of the 
mass. Moreover we find in many slaty rocks (in addition to the cleavage-planes) two sets 
of nearly vertical joints (one set, running parallel to the strike of the beds, I have called 
strike-joints; and the other set, which are nearly at right angles to them, dip-joints) which 
do accord with the mechanical conditions stated by Mr Hopkins, and probably represent 
them. The two names, dip-joints and strike-joints, | have frequently made use of, because 
of the very common occurrence of such joints*. 
In illustration of these views I will quote one example derived from the very coarse 
Cambrian conglomerates in the pass of Trebennau, on the old road between Llandovery and 
Pumsant. These conglomerates are highly inclined, and regularly alternate with great beds 
of coarse slates which have a distinct transverse cleavage. The conglomerates are cut by 
* Other less uniform breaks of continuity I have called diagonal-joints. They are of great irregularity; and I doubt 
not that multitudes of such joints have been formed by mechanical tension during the gradual desiccation and contraction 
of aqueous rocks, as well as by the gradual cooling and solidification of those which were partly of igneous origin. 
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