386 



cornubianite should be applied to the tourmaline-bearing contact-rocks, 

 and that the term proteolite should be revived for those consisting 

 essentially of quartz, mica and andalusite. If this suggestion were 

 adopted cornubianite would then be equivalent, or approximately 

 equivalent, to the tourrnaline-hornfels of continental petrographers and 

 proteolite would be equivalent to andalusite-hornfels. Confusion would 

 probably be saved if petrographers would consent to drop both terms 

 and content themselves with describing the rocks they meet with in 

 general terms. 



DE LA BECHE calls attention to the influence of granite on the sur- 

 rounding rocks, (1) and emphasizes the fact that each variety of rock has 

 its own special type of metamorphism. Thus he says : " Where the grits 

 of the carbonaceous series closely approach the granite on the north-west 

 of Dartmoor, as at White Hill, near Lidford, we find them taking the 

 character of quartz rock, and other instances of the same kind may be 

 observed from thence round by Okehampton. Wherever slates come into 

 contact with this granite, they become changed in appearance, some having 

 been rendered flinty .... In numerous localities we find the coarser 

 slates converted into rocks, resembling mica-slate and gneiss, a fact par- 

 ticularly well exhibited in the neighbourhood of Meavy, on the south-east 

 of Tavistock .... It would be uselessly occupying time to attempt a 

 description of all the altered rocks which occur in Cornwall, inasmuch 

 as the varieties are as considerable as the composition of the rocks brought 

 under the necessary conditions. The slates not unfrequently become 

 extremely hard and of a dark colour dark purple is by no means 

 an uncommon tint. Another common variety consists of a glossy grey 

 slate often containing disseminated and imperfect crystals of a mineral 

 resembling chiastolite." 



Mr. ALLPORT has described the microscopic structure of some 

 of the contact-rocks surrounding the Land's End mass of granite/ 2 ) 

 The rocks which he describes may be divided into two groups : 

 (1) those which contain tourmaline and (2) those which do not, 

 or those which contain it only in very small quantity. Examples 

 of both types may be obtained at Mousehole, near Penzance. The 

 rocks in question are essentially composed of quartz, brown or reddish 

 brown mica, white mica and tourmaline. They usually exhibit a foliated 

 texture, and are described by Mr. ALLPORT as tourmaline-schists and 

 mica-schists. The rocks are holo-crystalline and the individual con- 

 stituents exhibit no trace of a clastic origin. There has been a complete 

 recrystallization of the original constituents accompanied, at any rate in 

 the case of the tourmaline-bearing rocks, by the actual addition of material 

 (e.g., boracic acid). The phenomena are in many respects similar to those 

 described by Dr. HAWES in his paper on the New Albany granite. On 

 the west side of the Land's End mass of granite the contact-metamorphism 



(1) Report on Geology of Devon and Cornwall, p. 267. 

 2) Q.J.G.S., Vol. XXXII. (1876), p. 407. 



