PHYSICAL GEOLOGY. 175 



Structure of RockB, called Jointing ; and its llelation to Slaty 



Cleavage. Trans. R. Irish Acad. vol. xxv. (Science), pp. 605-662 ; 



4 plates. 



Considers that many cases of so-called slaty cleavage should in 



reality be referred to jointing. There appears to be a relation between 



jointing and mineral cleavage, which is also a superinduced divisional 



structure ; and the author does not accept the mechanical theory of the 



origin of the former. He points out a remarkable agreement between 



jointing and a phenomenon of terrestrial magnetism, and thinks it 



possible to imagine that, conditions being favourable, jointings induced 



at any given period might coincide in direction with the magnetic 



meridian of that time. E. T. H. 



Mackintosh, D. On the Origin of Slickensides, with Eemarks on 



Specimens from the Cambrian, Silurian, Carboniferous, and Triassic 



Formations. (Abstract.) Quart. Journ, Geol. Soc. vol. xxxi. pp. 



386, 387. 



Su^ests that slickensides may be due to partial fusion during the 



movement of one face of rock against the other. 



Malet, H. P. Basalt. Sci. Goss. no. 124, pp. 90, 91. 

 A short note against the igneous origin of Basalt. 



. Cleavage of Slate. Sd. Gos. no. 125, p. 119. 



Mallet, R. Some Observations on the Rev. 0. Fisher's Remarks on 

 Mr. Mallet's Theory of Volcanic Energy. Quart. Journ. Geol. Soc. 

 vol. xxxi. pp. 511-518. 

 See the following notice. 



. On the Temperature attainable by Rock-crushing and its 



Consequences. Phil. Mag. ser. 4, vol. 1. pp. 1-13 ; reprinted in 

 Amer. Jowm. ser. 3, vol. x. pp. 256-268. 

 In answer to Prof. Hilgard's and Rev. 0. Fisher's criticisms on his 

 Theory of Volcanic Energy, the author explains in what way the tem- 

 peratures consequent upon crushing the materials of the earth's crust 

 may be sufficient locally to bring these into fusion. A cube of rock 

 deep within a mass of similar rock will require a much greater force 

 to crush it than when exposed to pressure on two surfaces only (as in 

 the experiments), and the work done and heat produced will be pro- 

 portionally greater. At 20 miles depth, with an initial temperature 

 of 1000° Fahr., the heat developed by the work of crushing each cubic 

 foot of rock will fuse its own volume. To Mr. Fisher's objection that 

 he sees no reason why the heat developed by the crushing of 10 cubic 

 miles of rock should be localized so as to fuse 1 cubic mile, the author 

 argues that when a column of rock is pressed against an unyielding 

 mass, crushing will commence at the weakest place if it be not homo- 

 geneous, and if it be so will commence where the column is in contact 

 with a fixed mass. The subsequent effects are, that the extremity of 

 the column is continually crushed by a succession of movements, and 

 that the crushed fragments are surrounded by materials of a higher and 

 higher temperature, the first portion having the minimum temperature of 



