ON CRYSTALLINE AND SLATY CLEAVAGE 837 



brought before jou — bow tbe pressure is sufficient to pro- 

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

 tbe self- same law, wbose footsteps we trace amid tbe crags 

 of Wales and Cumberland, extending into tbe domain of 

 tbe pastry-cook and iron-founder; naj, a wbeel cannot roll 

 over tbe balf- dried mud of our streets witbout revealing 

 to us more or less of tbe features of tbis law. Let me 

 say, in conclusion, tbat tbe spirit in wbicb tbis problem 

 bas been attacked by geologists indicates tbe dawning of 

 a new day for tbeir science. Tbe great intellects wbo 

 bave labored at geology, and wbo bave raised it to its 

 present pitcb of grandeur, were compelled to deal witb 

 tbe subject in mass; tbey bad no time to look after de- 

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

 ing; facts are flowing in wbicb, wbile tbey leave un- 

 toucbed tbe intrinsic wonders of geology, are gradually' 

 supplanting by solid trutbs tbe uncertain speculations 

 wbicb beset tbe subject in its infancy. Greologists now 

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

 nature, and to produce ber results; tbey are approacbing 

 more and more to tbe domain of pbysics, and I trust tbe 

 day will soon come wben we sball interlace our friendly 

 arms across tbe 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 tbe lateral yielding, 

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

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

 to do wben tbis Lecture was given. Tbis sliding is, I 

 tbink, tbe principal cause of tbe planes of weakness, both 



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



Science — V — 15 



