38? 



is very marked about Cape Cornwall and Botallack. Here Mr. ALLPORT 

 observed both tremolite and andalusite ( 1) as accessory constituents in the 

 contact-rocks. The mode of occurrence of the tremolite in a rock from 

 Botallack is exceptionally interesting. It occurs in long bladed crystals 

 which often cross the planes of foliation without in any way affecting 

 them. Lines of black granules pass uninterruptedly through the 

 tremolite blades. Here we have undoubted evidence of the develop- 

 ment of idiomorphic crystals in a rock which has certainly never been 

 fused. The andalusite was observed by Mr. ALLPORT in a rock from 

 Cape Cornwall. 



The granite of Trewavas Head has produced important alterations, 

 similar in many respects to those described by Mr. ALLPORT. The 

 phenomena may be well studied at both the western and eastern junctions. 

 On the eastern side the " killas " has been traversed by broad bands of 

 granite, which are in no way affected by the powerful mechanical forces 

 that have crumpled and cleaved the former rock. The altered " killas " 

 preserves its fissile structure right up to the actual junction, except where 

 it has been converted into a rock composed of tourmaline and quartz. The 

 fissile rock is a highly micaceous slate which often presents a spotted 

 appearance (knoten-glimmer-schiefer). The constituents are quartz, white 

 mica, chlorite, andalusite, tourmaline and magnetite. The white mica 

 sometimes occurs in very large plates, which are mostly arranged with 

 their flat surfaces parallel to the planes of schistosity. The chlorite appears 

 to take the place of the brown mica so characteristic of contact meta- 

 morphism. It contains minute inclusions surrounded by pleochroic borders, 

 and is probably a secondary product resulting from the alteration of the 

 brown mica. Brown tourmaline is present only in very small quantity. 

 It occurs as minute crystals. The andalusite is not recognizable in all 

 slides. It is without definite crystalline form. The tourmaline-rock is 

 of a dark brown colour. It is composed essentially of brown tourmaline, 

 in grains or crystals, and quartz. Some varieties contain a considerable 

 amount of brown mica of the same colour as the tourmaline. The two 

 minerals may, however, be readily distinguished by the fact that the more 

 or less lath-shaped sections appear dark in opposite positions when the 

 stage is rotated over the polarizer. The relation of the tourmaline-rock to 

 the ordinary micaceous-slate is very interesting in certain places. It 

 (the tourmaline rock) is found on either side of joint-planes which traverse 

 the altered killas at right angles or approximately at right angles to the 

 schistosity. This relation is readily explained if we regard these joint 

 planes as having been the channels by which vapours, including boracic 

 acid, obtained access to the rock ; indeed it can be explained on no other 

 hypothesis. The veins which produce the rnetamorphism above described 

 are composed of a tourmaline-bearing muscovite granite, in which topaz 



(1) Mr. ALLPOET speaks doubtfully about the occurrence of andalusite. His slides, however, 

 show that in this instance he has been unnecessarily cautious. The mineral possesses the charac- 

 teristic pleochroism, refraction and double -refraction. 



