172 Mr. Hopkins's Reply to Dr. Boase's Remarks 



not commence at the surface but, at sojne lower part of the 

 mass. If the extensibility of the lower part of it be sufficiently 

 increased by its higher temperature, the fissures will com- 

 mence at some point between the upper and lower surfaces, 

 and will be propagated both upwards and downwards, and 

 may or may not, according to the degree of extension of the 

 mass, reach either the upper or lower surface. This is a point 

 of little consequence as far as regards Dr. Boase's objection 

 above stated, but it may serve to account for the fact that 

 eruptions of fluid matter in some cases have, and in others 

 have not, accompanied dislocations and elevatory movements. 



In my investigations I have spoken of elevatory forces, the 

 idea of the higher portions of the earth's crust having been 

 elevated being in general, perhaps, more familiar to us than 

 that of the lower having been depressed. I would here ob- 

 serve, however, that so far as relates to the first production of 

 fissures it is immaterial whether we suppose the mass to be 

 bent upwards by a force beneath, or downwards by its own 

 weight, provided the regions thus subsiding simultaneously 

 be of the same extent as those which I have always spoken of 

 as being simultaneously elevated. The secondary phaenomena, 

 however, resulting from the fissures produced by the up- 

 ward, and by the downward movements respectively, would 

 probably present certain characteristic differences ; but I shall 

 not now enter into any further investigation of them. 



In the abstract (Art. V.) of my memoir which appeared in 

 your Journal (vol. viii. p. 359), I have taken considerable pains 

 to indicate the possible influence of a jointed structure existing 

 in the elevated mass previously to its elevation, and how it 

 might be ascertained by observation, whether or not this influ- 

 ence had been considerable. Dr. Boase, however, is disposed 

 to arrive at the determination of this point by the shorter, but, 

 in such matters, most unsatisfactory road of a priori reasoning. 

 He observes: "If then solid rocks have necessarily a jointed 

 structure, one of the data on which Mr. Hopkins's calculations 

 are founded is invalidated, in as much as the elevating force 

 can never have acted on a solid mass without the interference 

 of this modifj'ing circumstance." Now, in the first place I 

 would observe that the process of solidification of all rocks 

 must necessarily have been extremely slow, and that probably, 

 therefore, all the modifications which they have undergone 

 during that process must have been the gradual work of 

 lengthened periods of time. It is impossible, then, to say 

 what period might be necessary for any portion of the earth's 

 crust to arrive at that state of its jointed structure which 

 should produce any decided effect on the directions of its dis- 



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