of the Structure of Rocks. 447 



masses, producing a beautiful crystalline cleavage, passing 

 alike through all the strata." There can be no doubt that the 

 particles of a solid rock may undergo a new arrangement by 

 the action of heat, if sufficient to overcome the previously 

 existing cohesion; as in the case of crystalline limestones 

 next igneous rocks, of sandstones which have been long sub- 

 jected to the heat of a furnace, and in similar instances; this, 

 however, is but a secondary or superinduced operation of the 

 attraction of aggregation, after its partial or total suspension 

 in solid masses by the superior force of an antagonist power. 



The Professor admits that igneous as well as stratified 

 rocks have also been subject to this crystalline action, for he 

 says (at p. 482.) that " even in formations of true granite we 

 occasionally see imperfect indications of a cleavage," which 

 " has, I believe, in most cases been produced during the pass- 

 age of the vvhole granitic mass into a solid state by that kind 

 of compound crystalline force which has produced the trans- 

 verse laminations of argillaceous schist." Although the indi- 

 cations of a cleavage are imperfect in granite, they are not so 

 in all igneous rocks, since some trappean rocks afford good 

 slates. A perfect cleavage, therefore, can in fact have no such 

 absolute connexion with the mode in which the materials of 

 a rock have been deposited as to make this a criterion 

 whether a rock has or has not a true cleavage, because many 

 rocks are stratified and yet are not fissile. We think, then, 

 that it must be admitted that the cleavage, both of igneous 

 and aqueous rocks, is a crystalline structure ; and that a rock 

 must be considered as fissile, as a true slate, when it cleaves 

 into thin laminae, whether these be transverse to the strata or 

 not. It remains to be seen whether geologists will sanction 

 the proposed innovation on the old nomenclature. 



The last kind of structure of which the Professor has treated 

 in the paper under consideration is the "jointed structure", 

 which he is of opinion has had a very different origin from 

 the fissile or slaty structure ; the latter having " resulted 

 from the ultimate chemical [query crystalline ?] arrangement 

 of the particles of a rock," whilst the former " seems in most 

 cases to have been produced mechanically, either by a strain 

 upon the rock from external force, producing, more or less, 

 regular sets of cracks and fissures*, or by a mechanical tension 

 on the mass (produced probably by contraction) during its 

 passage from a fluid or semifluid, into a solid state." 



It is stated (at p. 480. and 481.) that " many rocks, both 

 stratified and unstratified, are divided into solids of greater or 



* [We have printed these sentences as correctly quoted from the Trans. 

 Geol. Soc. by Dr. Boase, but we apprehend that there is some error in the 

 punctuation, and that they ought to stand thus, " producing more or less re- 

 gular sets of cracks and fissures." — Edit.] 



