HOOK NOTICES. 



planes, with usually some expansion along the cleavage-planes in the 

 direction of their dip. This compression has effected either a 

 flattening of the individual fragments constituting the rock, or, as 

 shown by Dr. vSorby, a re-arrangement of flat or elongated fragments 

 into positions nearly perpendicular to the compression, i.e., parallel 

 to what are now cleavage-planes. This peculiarity of internal struc- 

 ture gives the rocks their tendency to split into parallel plates quite 

 independent of any original lamination. This simple theory of 

 cleavage, which attributes it to the effect of lateral compression (not 

 merely lateral pressure), explains the fact, originally noticed by 

 .Sedgwick, that the general strike of the cleavage-planes in a district 

 is parallel to the main axes of disturbance. Mr. Mellard Reade, 

 however, laying stress on the fact that the cleavage is found to be 

 unaffected by, and therefore posterior to, the flexures of the rocks, 

 seems inclined, like Mr. Fisher, to refer the structure to "a constantlv- 

 sustained pressure for long periods and an after-relietV To our mind 

 the actual lateral compression of the rocks to a considerable extent is 

 proved by the distortion of the included fossils, and Prof. Haughton 

 has calculated the amount of compression in a number of cases from 

 measurements of their distorted forms. The origin of foliation is 

 a much more debatable question. The author apparently beheves 

 that foliation is superinduced in the rocks upon pre-existing structural 

 planes — bedding, cleavage, or fluxion-surfaces due to pressure. He 

 omits to notice the evidence obtained in recent years in Saxony. 

 Norway, Austria, and America, besides Sutherland and Cornwall, 

 that foliation may be produced in crystalline rocks by crushing or 

 ^flowing' caused by mechanical forces. 



The next chapter treats of earthquakes, which are here supposed 

 to be due to sudden horizontal expansion or contraction, or to 

 deeper-seated cubical contraction to which the overlying rocks can 

 adjust themselves only by subsidence. It is remarked that those 

 countries suffer most from earthquakes in which the newest deposit*; 

 are best developed. 



The remainder of the volume is devoted chiefly to working out 

 the general theory already indicated. The author regards mountain- 

 making as a slow process, accomplished by repeated expansions and 

 contractions of the underlying rocks. We think he does not succeed 

 in accounting very satisfactorily for the characteristic linear, or at 

 least axial, structure of mountain ranges ; nor, again, in establishing 

 their permanence, for we should expect as a logical complement to 

 his theory, that denudation would have the effect of lowering the 

 isogeotherms in the subjacent portion of the crust, and so causing 

 contraction and subsidence. 



May 1887. 



