PART II. CHAPTER XXIV. 



289 



Granite of Silurian Period. 



Silurian period. — I have already alluded to the granite near 

 Christiania, in Norway, as being newer than the Silurian strata 

 of that region. Its posteriority in date to limestones containing 

 orthocerata and trilobites, has long been celebrated, it being 

 twenty-five years since Von Buch first announced the discovery. 

 The proofs consist in the penetration of granite veins into the 

 shale and limestone, and the alteration of the strata, for a con- 

 siderable distance from the point of contact, both of these veins 

 and the central mass from which they emanate. (See p. 129.) 

 Von Buch supposed that the plutonic rock alternated with the 

 fossiliferous strata, and that large masses of granite were some- 

 times incumbent upon the strata ; but this idea was erroneous, 

 and arose from the fact that the beds of shale and limestone 

 often dip towards the granite up to the point of contact, appear- 

 ing as if they would pass under it in mass, as at a, Fig. 291., 



Fig. 291. 



Silurian. Granite. Silurian strata. 



and then again on the opposite side of the same mountain, as at 

 b, dip away from the same granite. When the junctions, how- 

 ever, are carefully examined, it is found that the plutonic rock 

 intrudes itself in veinsj and nowhere covers the fossiliferous 

 strata in large overlying masses, as is so commonly the case 

 with trappean formations.* 



Now this granite, which is more modern than the Silurian 

 strata of Norway, also sends veins in the same country into an 

 ancient formation of gneiss ; and the relations of the plutonic 

 rock and the gneiss, at their junction, are full of interest when 

 we duly consider the wide difference of epoch which must have 

 separated their origin. 



The length of this interval of time is attested by the following 

 facts : — The fossiliferous, or transition beds, rest unconformably 

 upon the truncated edges of the gneiss, the inclined strata of 

 which had been disturbed and denuded before the sedimentary 

 beds were superimposed. (See Fig. 292.) 



The signs of denudation are twofold ; first, the surface of the 

 gneiss is seen occasionally on the removal of the newer beds, 

 containing organic remains, to be scored and polished ; secondly, 



* See the Gcea Norvegica and other works of Keilhau, with whom I examined 

 this country. 



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