HORNBLENBE-SCHISTS, GNEISSES, ETC. OF SARK. 125 



interchanged. They run for considerable distances with much uni- 

 formity, just like alternating layers of dark clay and fine sandstone 

 in a stratified rock. Occasionally the light bands are more than 

 •75" thick, but both vary from about this thickness to mere lines. 



The dark bands, on microscopic examination, are found to con- 

 tain a considerable amount of felspar, and do not materially differ 

 from the corresponding parts of the ordinary hornblende-schist, 

 except in the presence of an occasional grain of quartz, which is 

 not common in the latter. The white bands, however, consist of 

 d( jmposed felspar, of a considerable proportion of quartz, and of 

 1 aer small flakes of a ferro-magnesian mica, somewhat altered, 

 \>. ii only an occasional grain of hornblende. These two rocks re- 

 semble in structure that of the granitic and hornblendic bands in 

 the Granulitic Group at the Lizard, the one passing sharply into the 

 other in exactly the same way. The dark bands also correspond 

 with the hornblende-schists of the latter region, but in these the 

 white bands, which are usually thinner and. frequently ^^as is some- 

 times the case here) not very well marked, consist, so far as we 

 know, mainly of felspar, and rarely contain quartz. Sphene (in 

 one case abundantly), apatite, and epidote occur in the Sark horn- 

 blende-schist, but we have not yet found the colourless augite of 

 the Lizard. The dominant hornblende and felspar occur in grains, 

 not seldom both slightly elongated, and showing a parallel ordering, 

 as is frequent at the Lizard, whilst occasionally the structure seems 

 on the yerge of becoming ophitic. 



(c) The Banded Gneisses. — By this name we may designate a 

 rather important group of rocks, evidently in close association with 

 the last, excluding, however, for the moment one or two rocks to 

 which the term is equally applicable, since, for reasons which will 

 appear, they require to be separately noticed. 



The group as a whole is characterized by more or less micaceous 

 bands, alternating with those which consist mainly of quartz and 

 felspar. The numerous varieties may be roughly grouped about 

 two types : the one, moderately coarse in texture, marked by rather 

 clearly defined bands, and fairly rich in biotite ; the other, finer- 

 grained and less micaceous, in which the banding is indicated 

 macroscopically as a difference of tint rather than by conspicuous 

 predominance of a particular mineral. The latter type, not un- 

 frequently, could be very nearly matched by specimens from the 

 Granulitic Group of the Lizard (though these usually are rather 

 redder in colour), but we have not met with the former there, although 

 one of the gneisses which occur in the islets south of Polpeor bears 

 some slight resemblance to it. Of the two types the coarser 

 appears to be the commoner in Sark. It is the dominant rock on 

 the north-eastern coast from the margin of the Eperqueries granite 

 to beyond La Greve de la Ville ; also in Derrible and Dixcart 

 Bays, and probably to the south, until the granite of Little Sark sets 

 in. On the western side it occurs in similar relations to the latter 

 rock, extending northwards by Port es Saies and Havre Gosselin 

 to the southern side of Port a la Jument. North of that, the finer- 



