ON THE CRYSTALLINE ROCKS 01' THE LIZARD DISTRICT. 47'3 



former mineral is probably a chlorite, similar to that described by 

 Herr Weigand. 



(2) From the intrusive junction in Pare Bean Cove (2 specimens) : 

 one, " 2 or 3 inches from contact-surface," consists of roundish or 

 rather oblong grains of pale-brown hornblende and colourless 

 augite in a minutely granular matrix ; this is composed of a filmy 

 brownish mineral, resembling the hornblende, and a colourless one 

 (? felspar), together with some scattered granules of brownish iron- 

 oxide. Some transverse cracks are filled by secondary minerals, in 

 part a flaky hornblende. The other specimen, " from a slab about 

 8 inches thick, enclosed in serpentine," is very like that from Porth- 

 alla, except that two or three bands in it are chiefly occupied by a 

 dull green chlorite, which in places is iron-stained, and is associated 

 with grains of magnetite ; also one or two grains of decomposed 

 felspar can be detected. 



(3) From a piece about the same thickness, similarly enclosed, 

 from the pit above Carnbarrow. This, to some extent, combines in 

 one specimen bands which respectively resemble parts of each of 

 the above, but other bands are characterized b}* a fair amount of 

 decomposing felspar, and thus in structure it more closely resembles 

 the normal rock. 



In two of these the above-mentioned mineral changes might 

 possibly be set down to ordinary decomposition, though we do not 

 so interpret it; but this could hardly be asserted in the first 

 and last specimens. The alterations, especially in the hornblende, 

 so far as our experience goes, resemble those which have been pro- 

 duced in " greenstones " by contact-metamorphism. In each case, 

 as it happens, the hornblende-schist is a well-banded variety. 



Mr. Collins states that " the massive serpentine of Porthalla does 

 not present that reticulated structure which has been regarded as 

 the result of the alteration of olivine rocks " *. Either he must have 

 examined a very small number of specimens and been singularly 

 unfortunate in collecting them, or have had a limited experience in 

 the structure of altered peridotites. It is quite true that there are 

 differences between this rock and the serpentine at Coverack or 

 farther to the south ; the latter contains more bastite, and as it was 

 once rather coarsely crystalline, the characteristic reticulated structure 

 can be seen at a glance. The Porthalla rock, however, was evi- 

 dently at the first a fine-grained peridotite f, but, making allow- 

 ance for this, it presents no difficulties to an experienced eye. Occa- 

 sionally the structure is as characteristic, though on a scale of about I, 

 as in the normal rock to the south. In some parts, where the 

 banding is most marked, there is considerably more variation in the 

 mineral constituents ; a white micaceous mineral in small flakes is 

 occasionally rather abundant — possibly the variety of chlorite 

 investigated by Herr Weigand in the Rauenthal serpentine — larger 

 flakes of this are sometimes associated in nests, with grains of mag- 



* Geol. Mag. (1885) p. 300. 



t Probably verv like that from St. Paul's Island, described by Prof. Kenard. 



Q.J.G.S. No.'l87. 2ic 



