SURROUNDING THE LAND's-END MASS OF GRANITE. 423 



of serpentine or the white snbstanee just mentioned; both are 

 highly characteristic ; the crystalline forms are perfectly preserved, 

 and they are traversed by veins representing the original cracks so 

 generally formed in this mineral. 



In one slice the angite is also greatly altered ; and it is interesting 

 to compare the two pseudomorphs side by side. 



Although the ground-mass of the rock exhibits a confused 

 amorphous appearance both in ordinary and polarized light, we are 

 fortunately not left in doubt as to the nature of at least one of its 

 original constituents. The augite frequently encloses highly cha- 

 racteristic pseudomorphs after felspar ; some are completely enclosed, 

 while others are only partially imbedded in it ; the mode of occur- 

 rence is, in fact, precisely the same as that observed in the altered 

 gabbro of Corstorphine Hill, near Edinburgh, and in several Scotch 

 dolerites described by me on a previous occasion*. In many cases 

 the unaltered augite has preserved in the most perfect manner the 

 sharp edges and angles of the felspar prisms ; and whenever the 

 latter project from the augite, it may be readily seen that both the 

 enclosed and outlying portions have been converted into precisely 

 the same serpentinous substance as that forming the ground-mass. 

 It should also be noted that, in the case of partially enclosed prisms, 

 only those sides or ends which have impressed their shape on the 

 augite exhibit a crystalline form, the outstanding portion being quite 

 undistinguishable from the surrounding mass. 



As it would evidently be absurd to suppose that originally there 

 was no felspar save that enclosed in the augite, there can be no 

 doubt that the original felspathic matrix has been completely meta- 

 morphosed, and that we have in the Menheniot rock a highly inter- 

 esting and instructive example of the conversion of an intrusive 

 olivine-dolerite into a mass of imperfectly formed serpentine. 



Near Torquay in Devonshire there are two masses of eruptive 

 rock exposed on the shore — namely, in Babbacombe Bay, and on the 

 south side of Anstis Cove ; they are intrusive in Devonian slates and 

 limestone, and like other rocks of the district are altered dolerites. 

 The mass in Anstis Cove is in the best state of preservation ; it is 

 rather coarsely crystalline, of a dark green colour, and consists of 

 augite, small prisms of triclinic felspar, apatite, and magnetite. 

 Some of the augite is but slightly altered ; there is a little pale-green 

 serpentine disseminated through the mass, and a few crystals of 

 epidote. One part of the rock is traversed by a vein which contains 

 numerous small crystals of this mineral enclosed in quartz ; the 

 clear quartz crowded with minute perfectly formed crystals of yellow- 

 green epidote is an exquisite microscopic object. Other portions 

 of the rock are so highly altered that there is nothing left but a 

 mass of pseudomorphs. 



In Babbacombe Bay there is an excellent section of the rocks, 



where the intrusive character of the trap may be well seen. There 



are here two principal varieties of dolerite : — one rather fine-grained 



and of a uniform grey colour ; the other of a lighter shade and por- 



* Quart. Journ. Geol. Soc. vol. xxx. p. 557. 



