Ch. IV.] SCHISTOSE GROUP OF CORNWALL. 



Fig. 2. 



Felspar Porphyry, at Mousekole, Cornwall. 

 (Ground Plan.) 



a very gentle angle. The porphyritic crystals of felspar are 

 sometimes large and distinctly defined, but more commonly 

 they have the usual imperfect forms of such crystals : they are 

 not confined to the central part of the course, but often abound 

 close to the junction ; and in many cases it is difficult to say 

 whether the slate or the compact felspar is the basis of this 

 rock : this is particularly the case when the seams or open 

 joints do not correspond with the line of junction, but cut off 

 a portion of the slate, as happens at every step, so that both 

 rocks compose the same individual mass or concretion, and 

 cannot be separated ' from each other. At A the course 

 divides into strings, so as to envelop portions of the slate 

 which, together with the elvan, are mixed in irregular elon- 

 gated portions, variously disposed : in several places portions 

 of rock are imbedded in the elvan, which perfectly resemble 

 the hard compact slate immediately adjacent to this course. 

 Some of the quartz-veins abut against the elvan, others tra- 

 verse it without any alteration ; but the vein B produces a 

 heave of twelve feet towards the north. On the western side, 

 the elvan is ten feet wide, gradually enlarging from eight ; 



E 4? 



