ON CRYSTALLINE AND SLATY CLEAVAGE 337 



brought before you how the pressure is sufficient to pro- 

 duce the cleavage. Expanding our field of view, we find 

 the self -same law, whose footsteps we trace amid the crags 

 of Wales and Cumberland, extending into the domain of 

 the pastry-cook 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 deal with 

 the subject in mass; they had no time to look after de- 

 tails. But the desire for more exact knowledge is increas- 

 ing; facts are flowing in which, while they leave un- 

 touched the intrinsic wonders of geology, are gradually 

 supplanting by solid truths the uncertain speculations 

 which beset the subject in its infancy. Geologists now 

 aim to imitate, as far as possible, the conditions of 

 nature, and to produce her results; they are approaching 

 more and more to the domain of physics, and I trust the 

 day will soon come when we shall interlace our friendly 

 arms across the common boundary of our sciences, and 

 pursue our respective tasks in a spirit of mutual helpful- 

 ness, encouragement and goodwill. 



[I would now lay more stress on the lateral yielding, 

 referred to in the note at the bottom of page 334-, accom- 

 panied as it is by tangential sliding, than I was prepared 

 to do when this Lecture was given. This sliding is, I 

 think, the principal cause of the planes of weakness, both 

 in pressed wax and slate rock. J. T. ; 1871.] 



