Rev. J. J. Cony be are on the Veins of St. Agnes. 403 



the drawing annexed.* It has been much worked for tin, which 

 is disseminated through its mass in veins apparently contempora- 

 neous, f The killas which is traversed and covered by these more 

 crystalline rocks, has, for the most part, the characters usually as- 

 cribed to clay slate, and its strata occasionally present singular cur- 

 vatures ; in many places it passes into chlorite slate, and in the 

 immediate neighbourhood of these dykes it usually presents either 

 a highly crystalline form of that rock, or such an intermixture of it 

 with quartz and felspar as might fairly be esteemed a variety of 

 gneiss. This change of appearance has it is well known been 

 attributed by the most able advocates of the Huttonian theory to 

 the action of the injected mass, while yet in a state of fluidity. To 

 us, the aspect of the rock at the point of contact did not, (either in 

 this or any other instance of the same phenomenon, which fell 

 under our notice), appear to be such as we could conceive to have 

 resulted from that process. 



We were in almost every instance strongly tempted to regard the 

 elvans as of contemporaneous formation with the schistose rock 

 which they traverse. We are conscious however that our obser- 

 vations were neither sufficiently accurate or extensive to warrant 

 the advancing any thing like a decided opinion upon this curious 

 subject. 



* PI. 23. Fig. 1. 



+ I have thrown into a Note the description of three specimens from different points of 

 this headland. 



1st. Small grained granite with earthy felspar, containing imbedded crystals of 

 flesh-coloured felspar. 



2d. The same, but with little mica, and the felspar somewhat less earthy. 



3d. A granite composed of middling sized grains of white vitreous quartz, light flesh- 

 coloured felspar, and a comparatively small quantity of dark brown mica. 



These, with several other specimens from the same quarter, arc deposited in the cabi- 

 nets of the Society. 



