124 



EEV. E. niLL AND PROF. T. G. BOKNET ON THE 



logical details which could have been easily inserted, because they 

 believe that these are repulsive to the ordinary geologist, and tend 

 to divert the attention even of experts from the main issues. 



(a) TJie Basement Gneiss. — Mr. Hill has already described the 

 macroscopic character of this gneiss,^ so that it is only necessary to 

 call attention to one or two details which are brought out more 

 clearly by study with the microscope. The felspar (plagioclase, 

 microcline, and probably orthoclase) is not idiomorphic, and seldom 

 shows in any part a rectilinear boundary. The outline of the 

 grains is more or less irregular or wavy ; they vary in diameter 

 roughly from "Ol" to '05" or a little more, about *03" being a 

 common size ; not seldom they seem to enclose small rounded 

 grains of quartz. The latter mineral, however, occurs commonly 

 in grains about the same size as the felspar, but often of a more 

 elongated form. The flakes of biotite (commonly altered into the 

 usual green mineral, with dark lines of iron oxide between the 

 cleavage-planes) are rather irregular in outline and usually about 

 •02" long. A slight tendency to aggregation in streak-like patches 

 is exhibited by all these minerals, but it is more conspicuous in the 

 quartz and the mica than in the felspar. We find occasionally a 

 grain exhibiting a micrographic intergrowth of quartz and felspar. 

 In short, the structure of the rock closely resembles that of the 

 more granitoid bands in the Granulitic Group at the Lizard, and, 

 as has been remarked of the latter, is very different from that of an 

 ordinary granite, being most nearly approached by some of the 

 fine-grained vein-granites. Indications of mechanical disturbance 

 subsequent to crystallization may be seen occasionally, but usually 

 are inconspicuous. 



(b) The Hornhlende-seliists. — For the macroscopic aspect of these 

 rocks we may again refer to Mr. Hill's paper. The more typical 

 varieties appear to occur in greater bulk along the western coast 

 of the island, and especially about Port du Moulin, from near 

 Saignie Bay to south of Port a la Jument. Most of these are so 

 like macroscopically to the hornblende-schists of the Lizard that 

 we have only examined a few specimens under the microscope. 

 These, both in structure and mineral constituents, correspond so 

 closely with the Lizard rock that it is enough to refer to the descrip- 

 tions which have been already published.^ Occasionally, however, 

 as in Port a la Jument, they are rather more distinctly and broadly 

 banded than at the Lizard, and this variety calls for a fuller de- 

 scription.^ In it bands consisting mainly of dark lustrous horn- 

 blende and of almost white quartzo-felspathic rock are repeatedly 



^ He called it, frona the locahty about which it is most developed, the ' Creux 

 Harbour gneiss.' We satisfied ourselves of its occurrence in the Burons and 

 other islets of this part of the coast. As a rock practically identical occurs in 

 the same position on the west coast, we now employ a more general term. 



2 Bonney, Quart. Journ. Geol. Soc. vol. xxxix. (1883) p. 1 ; McMabon, ibid. 

 vol. xlv. (1889) p. 519. See also ihid. vol. xlvii. (1891) p. 478, 



^ The specimen was broken from a large block, one of several at the base of 

 the crags in the southern bay at Port a la Jument, and the rock occurs, so far 

 as we could ascertain, rather low down in the thick mass of hornblende-schist. 



