738 



STRATA IN CONTACT WITH GRANITE. [Ch. XXXV. 



which at points remote from the granite is of an earthy texture and 

 blue color, and often abounds in corals, becomes a white granular 

 marble near the granite, sometimes siliceous, the granular structure 

 extending occasionally upwards of 400 yards from the junction ; the 

 corals being for the most part obliterated, though sometimes pre- 

 served, even in the white marble. Both the altered limestone and 



Fig. 757. 



Altered zone of fossiliferous slate and limestone near granite. Christiania. 

 The arrows indicate the dip, and the straight lines the strike, of the beds. 



hardened slate contain garnets in many places, also ores of iron, lead, 

 and copper, with some . silver. These alterations occur equally, 

 whether the granite invades the strata in a line parallel to the general 

 strike of the fossiliferous beds, or in a line at right angles to their 

 strike, as will be seen by the accompanying ground plan.* 



The indurated and ribboned schists above mentioned bear a strong 

 resemblance to certain shales of the coal found at Russell's Hall, near 

 Dudley, where coal-mines have been on fire for ages. Beds of shale 

 of considerable thickness, lying over the burning coal, have been 

 baked and hardened so as to acquire a flinty fracture, the layers being 

 alternately green and brick-colored. 



• The granite of Cornwall, in like manner, sends forth veins into a 

 coarse argillaceous-schist, provincially termed killas. This killas is 

 converted into hornblende-schist near the contact with the veins. 

 These appearances are well seen at the junction of the granite and 

 killas, in St. Michael's Mount, a small island nearly 300 feet high, 

 situated in the bay, at a distance of about three miles from Penzance. 



The granite of Dartmoor, in Devonshire, says Sir H. De la Beche*, 

 has intruded itself into the slate and slaty sandstone called graywacke, 

 twisting and contorting the strata, and sending veins into them. 

 Hence some of the slate rocks have become " micaceous ; others more 

 indurated, and with the characters of mica-slate and gneiss ; while 

 others again appear converted into a hard-zoned rock strongly im- 

 pregnated with felspar." f 



* Keilhau, Gsea Norvegica, pp. 61-63. 



f Geol. Manual, p. 4*79. 



