1823.] Mr. Moyte on Granite Yeini. 9l 



Trewavas Head, in the parish of Breage, in Cornwall, few of 

 which, in my opinion, can be clearly understood by those who 

 have never visited the spot, consequently less likely are they to 

 be able to decide on their disputed nature, whether the granite 

 composing the veins is of the primitive or secondary formation. 

 Having very recently visited the spot, and taken Mr./Sedgwick's 

 description of these veins with me, I find the account given by 

 him nearly correct ; at the same time I discover that he has 

 omitted to notice some circumstances which might tend to 

 elucidate, in a more correct manner, the nature of their forma- 

 tion. In endeavouring to supply this deficiency, I have thought 

 it advisable to give a section of the cliiF, or an outline of its 

 appearance from the beach at low water (PI. XXI), fig. 1, and 

 add a few observations which I conceive necessary as we pro- 

 ceed in his description. 



" About a quarter of a mile east of Trewavas Point (and about 

 100 yards east of this sketch), where the chfFs are in an unusu- 

 ally ruinous state, a small brook has excavated a passage to the 

 water's edge. The killas rocks on the beach appear to be inter- 

 sected by numerous contemporaneous veins of quartz. Near 

 this spot several thin beds of granite seem to alternate with 

 the slate ; one in particular, which preserves its thickness 

 and conformity to the lamina of the schist for upwards of 

 100 feet, when it is lost in the waters." The slate lying both 

 above and below this granitic vein as it traverses the beach, is 

 washed from its surfaces, so as to leave it projecting in many 

 places several feet, so that its dip is very visible, and is found to 

 be as in the cliff at about an angle of 28°. " However, a further 

 examination," says Mr. Sedgwick, " discovered its real nature ; 

 for upon observing it in an opposite direction, a number of smaller 

 veins were seen emanating from it. It then cut obhquely 

 through the lamina of slate, starting off from its first direction, 

 and became finally lost in a waving line among tlie chffs. The 

 greatest width of this vein is about two feet, and its extent from 

 the edge of the water to its termination in the cliff is about 400 feet. 



" Further west, the granite veins are crossed by two others 

 of a different character ; one of them ranges nearly in the 

 magnetic meridian, and underlies east two feet in a fathom ; the 

 other underlies in an opposite direction. They are about a foot 

 and half in width, and contain quartz, oxide of iron, and a Httle 

 clay slate." This quartz vein ranging nearly in the magnetic 

 meridian, produces upon the granite vein, the same effect that 

 cross courses often have upon metalliferous veins in most of our 

 mines, that of heaving it out of its direct course. Here, 

 fig. 2, the granite vein is heaved up about three feet by being 

 intersected by the quartz vein ; while another quartz vein, a few 

 feet distant, is seen j)ursuing its regular course, being interrupted 

 only by the granite vein. This circumstance I shall have occa- 

 sion to remark more fully hereafter. 



