1G4 PHYSICAL GEOLOGY. 



Geikie, Prof. A. On some Points in the connexion between 



Metamorphism and Yolcanic Action. Trans. Edin. Geol. Soc. 



vol. ii. part 3, pp. 287-307. 



Shows we might expect that metamorphism and volcanic action will 



]be found to go together, thus : — Metamorphism always goes along 



Avith intense crumpling ; and where there has been intense crumpling 



a shattered region is produced, uj) which internal heat will find its 



way, and down into which water will penetrate ; and these are 



conditions tending to produce volcanic action. 



Gives instances which seem to show that there is a connexion 

 between the two, and that periods of great metamorphism have been 

 also periods of great volcanic activity, dwelling specially on that fur- 

 nished by the metamorphosed L. Silurian of Scotland. Anticipates 

 the objection that volcanic action is superficial, while metamorphism 

 is believed to have been produced at great depths, by showing that 

 these depths have been overrated, bringing forward evidence that the 

 rocks of the Highlands could not have had a thickness of more than 

 5000 feet over them when they were metamorphosed. Having 

 shown that metamorphism is not a question of depth, the author argues 

 that it is related to crumpling, pointing out by way of instance that 

 the lowest parts of the L. Silurian of the north of Scotland, which 

 have been buried under the greatest thickness of rock, hut are not 

 crumpled, are but little altered, while higher parts of the series, 

 which were not so deeply buried, hut are intensely crumpled, are highly 

 metamorphosed; and adds that there are many rocks which have 

 had 10,000 to 12,000 feet of beds above them, and are yet unaltered. 

 Distinguishes between broad folding into troughs and arches and com- 

 plicated puckering : it is the latter that so persistently accompanies 

 metamorphism, while the former has often taken place without any 

 metamorphism being produced. It is probable that extreme meta- 

 morphism may end in the production of granitic rocks, and that when 

 a melted magma has been produced underground by intense metamor- 

 phic action, it will take the form of lava if it is forced out above ground, 

 and of intrusive granite if it harden underground ; and in support of 

 this view some new instances of the passage of granite through 

 elvanite into felstone are given. This leads to the belief that many 

 of the granite bosses in the Southern Uplands of Scotland are volcanic 

 necks, and the general conclusion that many of the so-called hypo- 

 gene rocks have not had the deep-seated origin usually ascribed to 

 them, but have been thrust up near to, if not quite to, the surface. 

 Many of the characteristic " Yesuvian minerals " are not found in 

 lava, but in blocks of altered rock, among the old tuifs of the moun- 

 tain ; and some of the same minerals occur among the metamorphic 

 rocks of Scotland — a fact which may be explained by supposing that 

 the metamorphism of the rocks surrounding Vesuvius is still in pro- 

 gress, and is one of the changes comprised within the circle of what 

 is called volcanic action. A. H. G. 



