PART I. CHAPTER XL 



143 



Alterations of Strata in contact with Granite. 



CHAPTER XL 



METAMORPHic ROCKS — contifiued. 



Strata near some intrusive masses of granite converted into rocks identical 

 with different members of the metamorphic series — Arguments hence derived as 

 to the nature of piutonic action — Time may enable this action to pervade denser 

 masses — From what kinds of sedimentary rock each variety of the metamorphic 

 class may be derived — Certain objections to the metamorphic theory considered. 



It has been seen that geologists have been very generally led 

 to infer, from the phenomena of joints and slaty cleavage, that 

 mountain masses, of which the sedimentary origin is unquestion- 

 able, have been acted upon simultaneously by vast crystalline 

 forces. That the structure of fossiliferous strata has often been 

 modified by some general cause since their original deposition, 

 and even subsequently to their consolidation and dislocation, is 

 undeniable. These facts prepare us to believe, that still greater 

 changes may have been worked out by a greater intensity, or 

 more prolonged development of the same agency, combined, per- 

 haps, with other causes. Now we have seen that, near the im- 

 mediate contact of granite veins and volcanic dikes, very extra- 

 ordinary alterations in rocks have taken place, more especially 

 in the neighbourhood of granite. It will be useful here to add 

 other illustrations, showing that a texture undistinguishable from 

 that which characterizes the more crystalline metamorphic form- 

 ations, has actually been superinduced in strata once fossilife- 

 rous. 



In the southern extremity of Norway, there is a large district, 

 on the west side of the fiord of Christiana, in which granite or 

 syenite protrudes in mountain masses through fossiliferous strata, 

 and usually sends veins into them at the point of contact. The 

 stratified rocks, replete with shells and zoophytes, consist chiefly 

 of shale, limestone, and some sandstone, and all these are inva- 

 riably altered near the granite for a distance of from 50 to 400 

 yards. The aluminous shale are hardened and have become 

 flinty. Sometimes they resemble jasper. Ribboned jasper is 

 produced by the hardening of alternate layers of green and cho- 

 colate-coloured schist, each stripe faithfully representing the ori- 

 ginal lines of stratification. Nearer the granite the schist oflen 

 contains crystals of hornblende, which are even met with in some 



