BOCK-M ETAMORPHISM , 



CHAPTEE XVI. 



THE CHRONO-GEOLOGICAL KANGE OF OPHITES AND RE- 

 LATED ROCKS, AND THE AGE OF THEIR METHYLOSIS. 



As, with certain exceptions already noticed, methylosis is con- 

 secutive to mineralization, it may be taken as granted that, 

 where both phenomena have prevailed, the former is of subse- 

 quent elaboration to the latter. 



Our remarks on this subject, however, must be brief, as it 

 is beset with no common difficulties, especially, as will have 

 been seen, in respect to the oldest or Archaean rocks. Indeed^ 

 amongst later systems insurmountable difficulties are encoun- 

 tered, as in the case of the ophites and hemithrenes of Ireland 

 and Scotland, which render it impossible to determine, with any 

 but the most distant approach to certainty, the geological age 

 in which these rocks passed through their last phase of meta- 

 morphism. And to refer to post-Archaeans of the kind in North 

 America, enough has been mentioned to show that not only the 

 period of their methylosis, but the age of their deposition is at 

 present involved in the greatest obscurity. 



Another difficulty attaches to rocks of a later age. In North- 

 ern Italy, methylosis has been developed in formations ranging 

 from an early period to the Jurassic ; and there are strong 

 grounds for believing that in this region the formations of one 

 system have undergone a change of the kind before those of a 

 later system had been deposited. The only certainty in con- 

 nexion with these rocks is that the latest change is of post- 

 Jurassic age. 



The ophitic marble of the Isle of Skye, which is Liassic, it 

 seems highly probable, has been mineralized and methylosed in 

 much later ages ; possibly the latest is synchronous with the 

 volcanic upbursts that have ravaged the western borders of 



