140 ILEV. E. HILL AND PEOF. T. G. BONNET ON THE 



micrograpliic. In this is a fair amount of biotite, with some white 

 mica in small flakes, the former being rather dirty-looking. The 

 rock is a quartz-felsite. The specimens from the other dykes do 

 not contain distinct grains of quartz, but, so far as can be deter- 

 mined without microscopic examination, they are quartz-felsites, a 

 little hornblende being probably present, at least in the Brecqhou 

 dyke. 



We also found on the south side of the western bay at the Coupee 

 a dyke, about 3 yards wide, of a moderately fine-grained granite of 

 a reddish colour.^ 



As Mr. Hill pointed out, dykes of greenstone abound in Sark, as 

 in the other islands. To their petrographical details a memoir 

 might be devoted ; they are generally rather compact,^ sometimes, 

 at the edges or in thin offshoots, so much so as to suggest a former 

 glassy condition; more rarely fairly coarse, not often porphyritic.^ 

 They are doubtless diorites or diabases (generally hornblendic), but 

 we have not thought them as a rule worth microscopic study, and 

 will only mention one, since it is slightly exceptional. 



The specimen is from a d)-ke which cuts the dioritic rock of Little 

 Sark (just mentioned), running up the cliff in Yermande Bay and at- 

 taining a thickness of at least 20 feet. It is a dark slate-grey (with 

 a faint reddish tinge) in colour, sufficiently coarse in texture to have 

 a slightly 'speckled' aspect, rough in fracture, with occasional 

 amygdules of rather irregular form and a dull sea-green colour. 

 Under the microscope we find that the groundmass exhibits an ophitic 

 structure, the plagioclase felspar occurring in crystals which average 

 about '06" in length ; the augite, which is in smaller grains and 

 less abundant, being generally rather decomposed and often replaced 

 by viridite. The larger felspars are very much decomposed, the 

 amygdules are irregular in outline and bordered by a thin zone of a 

 pale green chloritic mineral, and occupied mainly, if not wholly, by 

 calcite. 



The handsome rock intrusive in the granite at the Eperqueries 

 Landing deserves a little fuller notice than it has yet received.* 

 The porphyritic felspars in this rock are sometimes about half an 

 inch long, and do not diminish in size at the edges of the dyke. 

 Under the microscope, they are seen to be crowded with secondary 

 microliths, which make an exact determination of the species im- 

 possible, but probably most, if not all, are plagioclase. A fair 

 amount of biotite, more or less altered, occurs in small ground flakes, 



^ In Mr. Hill's collection is a moderately coarse grey granite which' he ob- 

 tained some years since on Little Sark in the part occupied by the above-named 

 diorite. Microscopic examination shows this to be' a true granite. We had 

 not time to search for it during our last visit, but we suspect this will prove to 

 be intrusive in the diorite. 



^ But, in many of these, examination with a lens shows them to be probably 

 holocrystalline, with a minute ophitic structure. 



^ There is a fairly porphyritic dyke on the S. side of Brecqhou, E. of the 

 Port ; but the most remarkable is the well-known dyke at the Eperqueries 

 Landing. 



^ As it also cuts through a greenstone dyke it must be a rather late in- 

 trusion. 



