Mr. George Tate on the Chemots. 367 



there are no changes in structural character seen, which can 

 be attributed to the action of a heated rock mass ; nor do the 

 porphyries and syenites penetrate as veins into the stratified 

 rocks with which they are in contact. It may, therefore, be 

 inferred, that, when these porphyries and syenites were erupted 

 through the stratified rocks, they were not in a highly heated 

 or even in a molten state. Still, however, they contain 

 ■within themselves pretty clear evidence of their igneous origin; 

 they are all more or less crystalline, and some of them are 

 entirely so ; and they rise as one great boss distinctly through 

 the unmetamorphosed stratified rocks by which they are sur- 

 rounded. In one sense they are metamorphic ; the result of 

 that normal or general metamorphism by which, far down in 

 the depths of the earth, mineral masses, whencesoever derived, 

 were by heat, or pressure, or vapours and gases, or by the 

 combined influence of all these several agencies, reduced to 

 a molten state, so as to admit of the free motion of the ele- 

 mentary substan(?es among themselves, and the development, 

 after cooling, of a crystalline structure. When thus in a 

 cool and consolidated state, these porphyries and syenites 

 have been, by the action of internal forces, protruded through 

 the superincumbent strata ; but the whole elevation, as we 

 shall see, did not take place at once, but, at least, in two 

 diiferent periods. 



After being erupted some slighter changes appear to have 

 taken place ; as the filling up of cavities and veins by quartz 

 crystals, chalcedonies, agates, and green earth, which was 

 most probably due to the infiltration of water charged with 

 silicates. The wavy ribboned structure of the agates around 

 a nucleus resemble much stalagmitic deposits from water satu- 

 rated with carbonate of lime. While, therefore, regarding 

 these porphyries and syenites as essentially igneous, it is not 

 necessary altogether to exclude the agency of water ; for even 

 modern volcanic eruptions of lava are accompanied by dis- 

 charges of heated water i and possibly, therefore, the hydrated 

 pitchstone may owe its peculiar structure to a similar acces- 

 sion of water when the rock was in course of consolidation. 



It may be admitted that some granites and porphyries, and 

 the gneiss into which they graduate, are metamorphosed strati- 

 fied rocks I but it would be unphilosophical to infer from this 

 that all other such rocks had the same origin; each case 

 must be tested by its own evidence. Now the Cheviot por- 

 phyries and syenites bear no trace of having been at any time 

 sedimentary rocks. No fragment of an organism appears ; 



3b 



