THE 



PHILOSOPHICAL MAGAZINE 



AND 



ANNALS OF PHILOSOPHY. 



[NEW SERIES.] 



MARCH 1829. 



XXIII. On the JunciioJi of the Grmiite and the Killas Rocks 

 in Corn'waU. By Messrs. Von Oeynhausen a7id Von 

 Dechen. 



[With a Plate.] 



T> EING in Cornwall we have paid great attention to inforiTi 

 -"-^ ourselves about the position of the granite and the killas 

 (argillaceous slate, hornblende slate, greenstone) ; never before 

 this time having an opportunity of seeing somuch of the junction 

 between these rocks as the romantic cliffs of this county show. 

 The granite forms several separated masses surrounded by 

 the killas, which we have seen in all the instances overlying 

 the former ; the strata of the killas are not, generally speaking, 

 parallel to the junction of this rock and the granite, but yet they 

 do not dip against this junction. Between Redruth and Cam- 

 borne the junction of these I'ocks is very well known in several 

 mines dipping to the north with an angle less than 45 degrees. 

 On the south of Carclase tin-mine near St. Austle, the junc- 

 tion between the granite and killas is nearly perpendicular, the 

 strata of the killas underlying very rapidly to the south. The 

 killas is mostly a well pronounced argillaceous slate of the 

 primitive class, and very like that of Johann Georgenstadt in 

 Saxony; but in the eastern and south-eastern parts of the county 

 it assumes the appearance of transition slate and of grauwacke. 

 In the immediate neighbourhood of the granite is found more 

 hornblende slate and greenstone than common argillaceous 

 slate. We cannot forbear to remark here, that the most part, 

 and also the richest, of the many lodes that occur in Cornwall, 

 both of copper and of tin, are found not very far from the 

 junction between the granite and the killas; the parish of 



• Communicated by the Authors. 

 N. S. Vol. r,. No. 27. March 1829. V 



Gwennnp, 



