1885.] 



Regional Metamorphism. 



429 



strata of the south of France are represented by massive 

 limestones and crystalline marbles. Normal metamorphism cannot 

 here be invoked, because it does not appear that these rocks have 

 been covered by any great thickness of newer rocks, or depressed to 

 such depths so as to bring them within the influence of the higher 

 underground temperatures. Nor can the change be always attributed 

 to contact metamorphism with granite or other eruptive or protrusive 

 rocks. It is only in cases where there is a central axis of any eruptive 

 rocks that this form of metamorphism can have acted, but in the 

 many cases where no eruptive rocks appear, the effect may, I would 

 suggest, be due entirely to regional metamorphism. 



The remarkable changes which take place in the condition of the 

 coal of Pennsylvania, as it ranges into the Appalachian Mountains, 

 may also be owing more probably to regional than to normal meta- 

 morphism. This mountain-range consists of a series of great parallel 

 folds, increasing in acuteness as the central axis is approached. 

 Eruptive rocks are absent ; but nevertheless the strata, as they 

 approach the central chain, become more crystalline, and the 

 coal, which at a distance is ordinary bituminous coal, passes into 

 anthracite, and even graphite. The late Professor H. D. Rogers 

 divided this great coalfield into four basins. The coal in the less dis- 

 turbed district near the Ohio river, where the flexures are extremely 

 gentle and wide apart, contains from 40 to 50 per cent, of volatile 

 matter ; in the wide basin further east it decreases to 30 or 35 per cent. ; 

 in the basins of the Alleghany range, in which, although there are no 

 important dislocations or great flexures, there are some extensive and 

 symmetrical anticlinical axes of the flatter form, the proportion of the 

 volatile matter in the coal varies from 16 to 22 per cent. ; while in 

 the most easterly chain of basins which are associated with the 

 boldest flexures and greatest dislocations, with close plications and 

 inversions of strata, the quantity of volatile matter in the coal is 

 reduced to from 6 to 14 per cent.* 



A somewhat analogous instance is presented by the Carboniferous 

 series of Belgium. The excessive squeezing, faulting, and inversions 

 which the Coal-measures have undergone on the flanks of the axis of 

 the Ardennes, is there accompanied by an alteration of the highly 

 bituminous coals into dry coals and into anthracite ; while the Car- 

 boniferous and Devonian limestones amidst the sharply convoluted 

 and folded strata of the Ardennes are there, as they are also on the 

 line of the same disturbance in the Boulonnais, transformed very 

 generally into semi-crystalline marbles. The few exposures of eruptive 



# Some geologists have referred the coincidence of these phenomena, partly to 

 the greater facility afforded for the escape of volatile matter when the fracturing of 

 the rocks has produced an infinite number of cracks and crevices, and partly to the 

 gases and waters which penetrated these cracks and promoted the disengagement of 

 volatile matter. 



