228 
THE GEOLOGIST. 
structure of rocks ; but notwithstanding tlie advance made by 
Prof. Sedgwick and Mr. Hopkins, there were points still re- 
maining to be investigated, before mathematicians could explain 
the force by which these phenomena had been produced ; because 
any such explanation must be based upon data afforded by the 
actual constitution of the rocks themselves. Twenty years since 
he had observed such remarkable symmetry in the crystalline 
forms of prismatic masses of slate in Westmorland, that he had 
measured their planes, and satisfied himself that they were not 
of the nature of ordinary crystallization, and that the term, 
crystalline structure, as usually applied to them, was by no means 
legitimate. In order to understand the nature of the mechani- 
cal forces which have produced the cleavage, it is important to 
study it in its relation to every form of strata. Where the con- 
tortions do not effect the cleavage planes, it is evident that they 
were first formed ; the joints and fissures which interrupt the 
cleavage but do not disarrange it, were probably posterior, like 
many of the faults which interupt the strata and cut off the 
cleavage. When layers of concretionary iron-stone occur in 
slaty rocks, the cleavage planes are arrested and '^troubled^^ in 
their passage through them, from which it appears that the 
nodules had become solid previously to the strata being divided 
by the planes of cleavage. Several attempts had been made to 
imitate cleavage structure. Mr. Fox, of Cornwall, had caused 
electric currents to pass through moistened clay, and had thus 
produced fissures parallel to the bounding surfaces of the mass 
and this illustration Mr. Phillips considered very important, 
but incomplete. The cleavage planes of the slate rocks of 
Wales were, he stated, always parallel to the main direction of 
the great anticlinal axis, but were not affected by the small 
undulations and contortions of those lines. In North Wales 
they maintain the same direction for fifty miles, not varying 
more than two or three degi-ees, which might be owing to local 
causes, such as the movement of masses by gravity at a period 
