G. F. Becker — Current Theories of Slaty Cleavage. 15 



of accounting for the frequent occurrence of mica on at least 

 one set of plane surfaces wherever the rocks affected by cleavage 

 have presented great resistance to deformation. Daring the 

 progress of a strain in a body which is not ideally brittle, 

 there is at first a cubical compression affecting the whole mass 

 alike and becoming constant at the elastic limit. Thereafter 

 an expenditure of energy takes place and this is confined to the 

 planes of maximum slide, where it is dissipated through the 

 viscosity of the material. It is not needful to prove this 

 statement, which is involved in the very definition of viscosity. 

 The temperature along these surfaces must rise, and, by increas- 

 ing the mobility of the molecules, the heat liberated will promote 

 chemical recombination in so far as compounds of lower poten- 

 tial are stable at the prevailing temperature and pressure. In 

 rotational strains the energy expended per unit mass along 

 those planes of maximum slide which are nearly fixed rela- 

 tively to the material particles will be many times as great as 

 along the set of planes which wanders rapidly through the mass, 

 and the chemical effects which may take place will be in some 

 direct proportion to this expenditure. Thus in slates due to 

 rotational strains it is comprehensible that one and only one 

 set of planes should be marked by abundant mica scales. In 

 irrotational strains there may be two or more mica-coated sur- 

 faces. On the other hand, there is no liberation of heat dur- 

 ing distortion which characterizes planes perpendicular to the 

 least axis of the strain-ellipsoid and none distributed along the 

 equipotential surfaces. 



A few pages of the memoir under review are devoted to a 

 discussion of strain and stress, and these call for some com- 

 ment lest they should mislead. The author states that when 

 the form of a body alone changes, the strain is called distor- 

 tion, which is in entire accord with usage. A few lines fur- 

 ther on he adopts from Yan Hise a term " pure shortening " 

 for " any irrotational strain in which all three principal axes 

 are changed in length in such a ratio that the volume remains 

 constant." As is well known, a "pure" distortion is synony- 

 mous with an irrotational distortion, so that shortening as here 

 employed is absolutely synonymous with distortion. Why so 

 well-chosen a term as distortion should be replaced by one 

 which does not suggest its definition is not clear. Shortening 

 is indeed absolute!}' misleading, for its opposite would be elon- 

 gation, a term frequently used in the discussion of strains, but 

 in a very special sense. It means extension in a single direc- 

 tion vnthout change of dimensions at right angles to this direc- 

 tion ; so that if the ratio of elongation is 2, the volume of the 

 mass is doubled. Elongation in this technical sense is merely 

 an analytical device. Only such cellular structures as pith, 



