of the Structure of Rocks. 451 



joints; but what, it may be asked, governed the fracture of 

 the rocks at right angles to these planes, for the cross joints 

 are as regular as the others ? And what cause produced the 

 third system of joints, in the cuboidal and rhomboidal masses, 

 which is, in general, but gently inclined to the horizon ? 

 Much stress has been laid on Mr. Hopkins's mathematical 

 investigations, which go far, it is said, to prove that " tabular 

 masses of rock elevated by a force from below, must have been 

 exposed to two sets of tensions, which would naturally pro- 

 duce longitudinal and transverse vertical fractures, at right 

 angles to each other." But this does not account for the third 

 system of planes, and more particularly for that which arises 

 from the division of rocks into rhombohedral solids, which 

 happens nine cases out of ten in regularly jointed rocks ; in* 

 deed, if we have not misunderstood Mr. Hopkins, the forma- 

 tion of such fractures by the tension of the mass from an in- 

 ternal force is a physical impossibility. 



As regards the production of 6C cracks and fissures" by the 

 other mechanical action mentioned by the Professor, viz. by 

 contraction, occasioned by the consolidation of rocks, it is not 

 very evident that this is a sufficient cause. Even in the desic- 

 cation of moist clay or starch, often quoted by other writers, 

 contraction seems only to be the effect of the aggregation of 

 the particles into less space, that is, the consolidation is ac- 

 companied by contraction ; but this is not the cause by which 

 the mass is divided into definite forms. It is doubtful, how- 

 ever, whether such open spaces would occur in sedimentary 

 deposits whilst under great pressure, and liable to the intro- 

 duction of additional aqueous sediments from the superincum- 

 bent unconsolidated mass. Much less can it be admitted that 

 masses which had been in a state of fusion would be rent into 

 cross fissures by the reduction of temperature ; in this case, 

 concretionary joints would certainly abound, and might even 

 be visible on a section of the mass; but it does not necessarily 

 follow that these joints should be open, or fissures, for crystal- 

 line bodies formed by fusion do not exhibit such appear- 

 ances. 



For the reasons which have now been advanced, I am in- 

 duced to withhold my assent from the views promulgated by 

 Professor Sedgwick concerning the structure of rocks. In- 

 deed they appear to me to have rendered the subject more 

 complicated than it was previously, by making unnecessary 

 distinctions. I may possibly be in error; and if so, I trust 

 that sound arguments will find me open to conviction. 



When the structure of igneous rocks was under considera- 

 tion, I proposed to divide their kinds of structure under three 



3 M2 



