50 DESCRIPTION OF THE PRIMARY [Ch. IV. 



granite and the slate ; but it is much better exhibited on the 

 shores of Mount's Bay, where several beds of felsparite traverse 

 compact and schistose species of cornubianite and actynolite 

 rock, on the beach below high-water mark. One of these, 

 near Penzance, is particularly deserving of attention; as a 

 considerable portion of it is exposed to view, and is easily 

 accessible. Immediately at the back of the pier, it may be 

 seen stretching N. E. and S. W., and inclining at a great 

 angle northward. It is a porphyritic felsparite (felspar por- 

 phyry), the imbedded crystals being small and v/hite : these 

 gradually disappear towards the sides of the bed, till at last 

 this rock is a plain compact felsparite, very hard, apparently 

 in consequence of an excess of silica in its composition ; and 

 in this part, veins of quartz are frequent, as is also the case 

 in the adjoining stratum of compact actynolite-rock, at the 

 point of their junction, where these rocks can only be dis- 

 tinguished from each other by the blue colour of the latter. 

 This granitic bed is lost eastward, under the sea, and west- 

 ward, under houses and the little neck of land which runs 

 towards the battery ; but on the other side of this small pro- 

 montory, it again makes its appearance, and may be examined 

 for nearly a thousand feet in length. It first emerges from 

 under the sand-bank near the chimney rock, a prominent part 

 of this bed ; the highest part of which consists of a single 

 prismatic block. The ground plan (fig. 1.) will serve to 

 illustrate its position, and its mode of connection with the 

 adjoining rock. 



The part figured is situated between the chimney rock and 

 the eastern extremity next the sand-bank, and is about two 

 hundred feet in length : in some points the line of junction 

 between the el van and slate bears N. E. by E., or even N. E., 

 as at the back of the pier ; the general direction of the whole 

 course, however, is E. by N, and W. by S., and the occasional 

 departures from this are apparent in the plan. On the 

 southern side the junction may be traced nearly two hundred 

 feet farther; on the northern side, with little interruption, 



