ON CR Y8TAL LINE AND SLA TY CL EA VA GE. 243 
perfectly blend together. The process of rolling draws 
them into fibers. Here is a mass acted upon by dilute 
sulphuric acid, which exhibits in a striking manner this 
fibrous structure. The experiment was made by my 
friend Dr. Percy, without any reference to the question of 
cleavage. 
Break a piece of ordinary iron and you have a granular 
fracture; beat the iron, you elongate these granules, and 
finally render the mass fibrous. Here are pieces of rails 
along which the wheels of locomotives have slidden; the 
granules have yielded and become plates. They exfoliate 
or come off in leaves; all these effects belong, I believe, to 
the great class of phenomena of which slaty cleavage forms 
the most prominent example.* 
We have now reached the termination of our task. You 
have witnessed the phenomena of crystallization, and have 
had placed before you the facts which are found associated 
with the cleavage of slate rocks. Such facts, as expressed 
by Helmholtz, are so many telescopes to our spiritual 
vision, by which we can see backward through the night 
of antiquity, and discern the forces which have been in 
operation upon the earth's surface 
Ere the lion roared, 
Or the eagle soared. 
From evidence of the most independent and trustworthy 
character, we come to the conclusion that these slaty masses 
have been subjected to enormous pressure, and by the sure 
method of experiment we have shown and this is the only 
really new point which has been brought before you how 
the pressure is sufficient to produce the cleavage. Expand- 
ing our field of view, we find the selfsame law, whose foot- 
steps we trace amid the crags of Wales and Cumberland, 
extending into the domain of the pastrycook and iron- 
founder; nay, a wheel cannot roll over the half-dried mud 
of our streets without revealing to us more or less of the 
features of this law. Let me say, in conclusion, that the 
spirit in which this problem has been attacked by geologists, 
indicates the dawning of a new day for their science. The 
great intellects who have labored at geology, and who have 
raised it to its present pitch of grandeur, were compelled to 
*For some further observations on this subject by Mr. Sorby and 
myself, see Philosophical Magazine for August, 1856. 
