102 ME. J. PARKINSON ON THE PTROMERIDES [Pob. 1 898, 



Vicart Point (which may be considered as forming the western 

 boundary of Boulay Bay) is composed, on its southern face, of rock 

 essentially similar to that just described from the back of L'Islet. 

 The top of the cliff consists of a dark drab or brownish-coloured 

 felstone, with light irregular pinkish veins or streaks, and containing 

 porphyritic quartz-crystals. La Tete des Hougues, which is the 

 easternmost point of the Bay at which the igneous rock is found, 

 is interesting, not only from the presence of large pyromerides with 

 exceptionally big cavities, but for the varieties of the rhyolite 

 which it shows. One of these is a red-brown rock characterized 

 macroscopically by a quantity of porphyritic felspars. These, 

 somewhat altered, are orthoclase. The rock contains quartz-crystals 

 rather plentifully, in some cases irregular in shape. It is also 

 interesting to note the presence of a few very bleached mica-crystals. 

 By incident light a section appears brick-red, and the flow- structure, 

 which is well marked, is of the familiar gnarled or wavy type. 



On the beach behind L'Islet, the reddish felstone mentioned above 

 apparently passes into a pale green rock, which resembles a true 

 frao-mental, owing to the presence of very numerous ' inclusions,* 

 o-enerally pinkish in colour ; these, owing to their superior hard- 

 ness, weather out of the softer rock in which they are set. 



A similar rock occurs near the jetty, a few hundred yards away, 

 showing practically the same arrangement of ' matrix ' and red or 

 purple fragments. These may be of large size, say 3x2 feet, and 

 weather so as to form ridges in their softer surroundings. They 

 are occasionally found with very irregular outlines and tongue- 

 like ends, suggestive of the softening and drawing-out so often seen 

 in fragments, similar in nature to the lava, which have been included 

 in the flow. 



Moreover, the rock contains other and dark green patches repre- 

 senting those portions of the magma itself, which, by some such 

 reason as local cooling or a slightly different composition, have 

 become individualized from their surroundings. Such facts point 

 either to a remarkable case of differentiation of one magma, or, 

 what may be more probable, the mixing of two in different stages 

 of solidification. 



An almost identical rock occurs immediately south of the jetty, 

 and may be briefly noticed now. Macroscopically, it is of a light 

 o-reen colour, showing small darker patches and numerous sub- 

 ano-ular fragments ; while microscopically, as might be anticipated, 

 it shows a great resemblance to a true fragmental rock. The 

 structure in thin section is obscured, and its interpretation ren- 

 dered more difficult by the presence of a considerable quantity 

 of a pale green mineral, almost inert with one nicol, and with two 

 o-iving tints of about the middle of the first order. The slide 

 contains numerous fragments (say '2 x '15 inch as an average 

 size), which, although not differing greatly in colour and appearance 

 from the rock in which they lie, are distinguished without difficulty, 

 more especially as they show occasionally an attempt at a flow- 

 structure. These help to produce the resemblance to an agglomerate ; 



