Ch. II.] 



TRANSITION FORMATIONS. 13 



respecting the nature of granite. First it was shown, by nume- 

 rous examples, that ordinary volcanic dikes might produce great 

 alterations in the sedimentary strata which they traversed, caus- 

 ing them to assume a more crystalline texture, and obliterating 

 all traces of organic remains, without, at the same time, destroy- 

 ing either the lines of stratification, or even those which mark the 

 division into laminae. It was also found, that granite dikes and 

 veins produced analogous, though somewhat different changes ; 

 and hence it was suggested as highly probable, that the effects to 

 which small veins gave rise, to the distance of a few yards, might 

 be superinduced on a much grander scale where immense masses 

 of fused rock, intensely heated for ages, came in contact at great 

 depths from the surface with sedimentary formations. The slow 

 action of heat in such cases, it was thought, might occasion a state 

 of semi-fusion, so that, on the cooling down of the masses, the 

 different materials might be re-arranged in new forms, according 

 to their chemical affinities, and all traces of organic remains might 

 disappear, while the stratiform and lamellar texture remained. 



May be of different ages. According to these views, the 

 primary strata may have assumed their crystalline structure at 

 as many successive periods as there have been distinct eras of 

 the formation of granite, and their difference of mineral com- 

 position may be attributed, not to an original difference of the 

 conditions under which they were deposited at the surface, but 

 to subsequent modifications superinduced by heat at great 

 depths below the surface. 



The strict propriety of the term primitive, as applied to gra- 

 nite and to the granitiform and associated rocks, thus became 

 questionable, and the term primary was very generally sub- 

 stituted, as simply expressing the fact, that the crystalline 

 rocks, as a mass, were older than the secondary, or those which 

 are unequivocally of a mechanical origin and contain organic 

 remains. 



Transition formations. The reader may readily conceive, 

 even from the hasty sketch which we have thus given of the 

 supposed origin of the stratified primary rocks, that they may 



