142 EEV. E. HILL AND PEOF. T. G. BOIS^JSTET ON THE 



in such a mess." Alteration of the original constituents has been 

 carried much further than in the boulder, but there is no reason to 

 doubt their general identity, for it is well known that these very 

 basic rocks sometimes vary considerably in different parts of the 

 same mass. 



We also found, on the shore at Port a la Jument, one or two 

 large boulders which correspond more closely with the rock of the 

 intrusive mass,^ though they are more distinctly porphyritic, large 

 patches of rather silvery pale hornblende occurring like the well- 

 known schiller-spar from Baste. 



As the outcrop which we found is so limited in extent, and as 

 Port du Moulin is bounded by projecting crags or skerries, we 

 suspect that there must be other outcrops of this picrite below 

 low-water mark.^ 



Mr. Hill mentioned the occurrence of a dyke of kersantite at 

 Port du Moulin, with traces of another. We can now increase the 

 number of mica-traps by four. One forms a small dyke in the 

 cliffs on the W. side of the Coupee. Another occurs in Saignie 

 Bay, running up the cliff just north of the narrow track 

 leading down to the shore, on which it also crops out. It is about 

 four feet wide, and, where best preserved, is a very fine-grained 

 dark rock, weathering brown, at first sight not unlike a basalt, 

 but with a slightly rougher and more glittering surface, owing to 

 the presence of many minute scales of mica. 



A third instance is a dyke between the two northern entrances ot 

 the Gouliot caves. In the cliffs it appears to be only about half a 

 yard thick, but it seems to broaden out on the rocky shore, and 

 may perhaps attain to six feet. It is a very characteristic, dark- 

 grey, slightly speckled mica-trap, and the mica crystals are more 

 conspicuous than in the last rock. 



A more curious variety occurs by the side of the beach in the 

 inmost recess of Havre Gosselin. Here a rather foliated dioritic 

 rock, mottled red and green, is intrusive in the usual hornblende- 

 schist, which is rather gneissoid in character. The former is cut 

 by a dyke of compact greenstone, and the two are severed by a 



^ There is a very strong macroscopic resemblance, but in the slice examined 

 from the Port a la Jument specimen there is no 'proof that ohvine has formed 

 one of the constituents ; hornblende, as above desci'ibed, occurring in a sort of 

 paste of minutely crystalHzed secondary products. The brown grains do not 

 occur here. 



2 In the description published in the ' Greological Magazine ' it is suggested 

 that the rock might come from 'the important vein of serpentine and steatite, 

 with asbestos and talc,' which, according to Prof. Ansted, ' has been traced 

 crossing the central part of Sark near Port du Moulin.' From the above 

 description it will be obvious that this rock cannot be the one mentioned by 

 Prof. Ansted, and we are unable to make out to what rock he is referring. 

 Probably the authority for his statement is a passage in Macculloch's paper 

 on the Channel Islands (Trans. Geol. Soc. vol. i. (1811) p. 18). The words of 

 the latter observer seem to imply that the dyke occurs in Port du Moulin. We 

 think no such dyke can now be seen, and as his visit was a hurried one we 

 should suppose some accidental error, if he did not speak of a Iwpis ollaris 

 being obtained above the cliifs and used by the inhabitants. It is possible 

 that some old excavations in the woods or fields may have escaped our notice. 



