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LXII. Some Facts 'w/iich appear to be at Variance isoith the 

 Igneous Hypothesis of Geologists. By Robert W. Fox*. 



^^HE speculations respecting the origin of rocks, and the 

 -■■ confidence with which their existing arrangements are 

 sometimes attributed to the agency of fire, induced me to en- 

 deavour to ascertain their expansion when heated ; and the 

 following are the results of the few experiments I have made. 



Pieces of granite increased in bulk when raised to a dull red 

 heat between j'jjth and g'ijth part ; and contracted on cooling 

 to their original dimensions. I did not detect any difference ia 

 these respects whether the granite was measured in the direc- 

 tion of its cleavage, or at right angles to it. At a full red heat 

 decomposition commenced, and vitrification at a white heat. 



Porphyritic felspar, from an " clvan course" heated to red- 

 ness, expanded jV to jL of its original dimensions; to which 

 it again contracted on cooling. 



Different specimens of clay-slate were augmented in size, in 

 the direction of their cleavage, -^j to Jj, by a heat scarcely 

 visible in the dark in some instances, and by a full i"ed in 

 others ; and when cooled, some of them were found to be per- 

 manently enlarged to nearly one half of the extent of the ex- 

 pansion the heat had produced. I could not clearly ascertain 

 the expansion of slate at right angles to its cleavage, from its 

 liability to split, but I think it was less considerable. 



Greenstone, at a red heat just visible, increased ^'^ or there- 

 abouts, and contracted back to nearly its previous bulk when 

 cooled. Serpentine, however, underwent no expansion in any 

 direction that I could appreciate, even when the heat was 

 raised to a full red. 



If, then, any of the rocks which are expanded so much by 

 great heat, had their origin fi'om irruptions of matter in igneous 

 fusion, ought they not to abound with fissures in every direction, 

 or at least to afford evidence of their having once existed in 

 them, iiidependcntli/ of other contiguous rocks? This consequence 

 seems to follow from their difierent expanding and contract- 

 ing pi'operties, even without adopting the hypothesis of various 

 epochs of formation. Such evidences, however, do not exist in 

 Cornwall at least ; but, on the contrary, our mineral veins, as is 

 well known, traverse all the rocks without anj' necessary change 

 in their size or direction. There are, it is true, frequent in- 

 stances of the thickness, and other characters of veins being 

 altered in passing from rocks of any given denomination into 

 those of another ; but if in some cases they become enlarged, in 

 others it is the reverse, so that no rule can be laid down in this 

 respect. Besides, there is far too great a conformity in the 



* Communicated by the Author. 



