474 PKOF. T. G. IJOXXEY AND MAJOR-GEN. C. A. M'^MAHON" 



netite, or lie nearly parallel (with inclusions of the latter) so as to 

 suggest the bleaching of a biotite *. Occasionall}'' the remains of a 

 pyroxenic constituent can be detected ; sometimes it is probably an 

 enstatite, but sometimes the oblique extinction of a remnant sug- 

 gests a colourless hornblende. In some cases a considerable quan- 

 tity of a semi-transparent mineral is present in minute granules, 

 occasionally aggregated, which suggests the presence of an alumina- 

 silicate. The banding is seen to be caused by the variation in the 

 amount of this mineral, the opacite, and the chlorite. The most 

 strongly banded variety, as we believe, is rather raref. The 

 structure in both is better seen in slightly weathered specimens than 

 on freshly broken surfaces. 



No sign of crushing can be discerned in these specimens. Both 

 the variety with thin streaky lines and little rounded eyes of a 

 mica-like mineral, and that with distinct bands of different colour 

 and texture J, present macroscopically a very close resemblance to 

 the fluxion-structure of a felstone or a rhyolitic rock. A very fine 

 specimen exhibiting this structure (which is much more conspicuous 

 on slightly weathered surfaces) is figured on Plate XVI. 



The microscopic examination of thin slices shows that the appa- 

 rent foliation is due to the streaky condition of the parent rock 

 prior to its serpentinization — differences in the original composition 

 of the streaks being now represented by slight mineral and struc- 

 tural differences in the resulting serpentine. In the opinion of the 

 authors, the structure can only be explained as a fluxion-structure ; 

 that is to say, as being the result of traction acting on either an 

 imperfectly blended mixture of two magmas, differing slightly from 

 each other in composition, specific gravity, or fluidity, as in the case 

 of a banded felsite or rhyolite, or on a mass, in which complete 

 crystallization had been arrested by subsequent motion at a time 

 when only a portion of the constituent minerals had separated 

 themselves out from the magma. 



The eruptive character of the serpentine (peridotite), which has 

 been described above, as well as the microscopic evidence, shows 

 conclusively that the original rock cannot have had a sedimentary 

 origin. 



(3) The Structures of the Serpentine. — A somewhat similar 

 structure is exhibited by the serpentine in other parts of the 

 Lizard district. Along the western coast this rock very commonly 

 exhibits some approach to mineral banding. For instance, at MuUion 

 sometimes, and rather more markedly at Lower Predannack, the 

 crystals of colourless hornblende tend to lie parallel. But, on 

 microscopic examination, we cannot discover that the constituent 

 minerals exhibit any indication of having either been crushed or 

 suffered any mechanical disturbance which cannot be explained 



* As in the scyelite of Caithness, Judd, Quart. Journ. Geol. See. vol. xl. (1884) 

 p. 406. 



t One specimen, picked up on the shore by Prof. Bonney, suggests the 

 possibility of the one variety being intrusive in the other (see Plate XVI. at the 

 line AB). J They are sometimes ^" wide. 



