of the Structure of Rocks. 379 



constituent particles of rocks. This kind of structure occurs 

 in trap-rocks of all ages, in granite, and in the primary slates, 

 which are also most probably of igneous origin. 



The following, therefore, may be the routine by which 

 rocks in a state of fusion have, on cooling, been consolidated. 

 First, the molecules attract each other to form the component 

 mineral or minerals; in the next place, these elementary mi- 

 nerals so unite together as to produce the various concre- 

 tionai forms; and lastly, the whole mass, on becoming solid, 

 acquires certain determinate figures, bounded by planes or 

 joints *. On this view of the subject, the various kinds of rock- 

 structure might be classed under three heads : the molecular, 

 the concretional, and the jointed or crystal-like structures. 



That the igneous rocks have been consolidated in the above- 

 mentioned manner, appears to be indicated by the facts, that 

 distinct crystals of the constituent minerals are often inter- 

 sected both by the concretional and jointed lines of structure; 

 and that, in like manner, the bounding planes of the crystal- 

 like masses traverse the various concretional forms. 



I have in another place entered into minute details on this 

 subject, and have adduced the fact of the same jointed struc- 

 ture being reciprocally common both to the granite and the 

 slate at their junction, as additional evidence in favour of 

 the contemporaneous origin of these rocks. The Professor 

 admits (at p. 483.) that the alternate layers of shorl-rock and 

 granite in St. Austel Moor indicate a crystalline arrange- 

 ment ; that " the whole rock has a laminated or veined struc- 

 ture, produced by a peculiar segregation of parts, in passing 

 from a state of fusion into a solid state." Again, " that the 

 alternations of these rocks become more frequent as we ap- 

 proach the junction of the slate, and at last are so frequent 

 and fine-grained, that the rock on the south side of Carclaze 

 becomes finely laminated, and passes into a true schist." And 

 then he asks, " What ought we to infer from a phaenomenon 

 like this?" and replies, "that the slate in contact with the 

 granite had at one time been nearly in the same condition as 

 the granite; and that both had been modified by a similar 

 crystalline action, in passing into a solid state." 



Now, this admission in the case of the granitic rocks of 

 St. Austel Moor, is a most important one towards the con- 

 firmation of my views ; for if the alternate layers and lami- 

 nated structure be the result of crystalline action in this in- 



* On this particular subject we may refer Dr. Boase and our geological 

 readers to a paper by Mr. Brayley in Phil. Mag. and Annals, N.S , vol. 

 viii. p. 331, in which some of the late Dr. Macculloch's views respecting 

 it are examined and extended. — Edit. 



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