422 



the surrounding sedimentary rocks. If porphyritic crystals are present, as 

 in the Ardennes' porphyroids, these may be more or less rounded and 

 cracked at right angles to the direction of stretching, like the belemnites 

 in the Jurassic rocks of the Alps. Secondary minerals, especially mica, 

 have been developed on the planes of schistosity. 



The schistose character becomes more and more marked as the 

 junction is approached, and the actual junction is more or less obscured 

 b}^ a migration of constituents and the development of similar structures in 

 both rocks. In short, the sequence, so far as structural features are 

 concerned, as we pass outwards from the centre of the dyke or sheet 

 into the surrounding sediments, is similar to that observed on a large 

 scale in passing outwards from a central mountain-axis. The most perfect 

 schists occur at the junctions of the two dissimilar kinds of rock, and the 

 actual plane of junction becomes more or less obscure. In regions of 

 intense dynamic metamorphism the original relations of the dyke or 

 sheet to the surrounding rocks are often completely destroyed, and 

 secondary schistosity, identical with that in the enveloping sedimentaries, 

 pervades the entire mass of the igneous rock. 



The case here referred to illustrates very well the general effect 

 of regional metamorphism. This agency develops a secondary parallel 

 structure in all rocks affected by it, whether igneous or aqueous, and 

 causes a complex of rocks of different ages and characters to simulate 

 a stratigraphical sequence. The strike and dip of the foliation-planes is 

 determined, in the first instance, by the distribution of the stresses which 

 produced the deformation, but is of course liable to be modified by later 

 earth-movements. The theory of dynamic metamorphism, now rapidly 

 gaining ground, is an extension of the views advocated long ago by 

 Darwin/ 1 ) Scrope/ 2 > and Sharped 



(1) Geological Observations on South America, p. 162 168, 1846. See also Geological 

 Observations, 2nd Edition, 1876, p. 460469. 



(2) Geologist, No. IX., 1858, p. 361. See also Volcanoes, 1862, p. 298. 



(3) Phil. Trans., 1852, p. 445. 



THE END. 



Jiemrose & Sons, Printers, 23, Old Bailey, London, E.C. ; and Derby. 



