322 ME. J. PAEKINSON ON THE ROCKS OF [May 1900^ 



measure *04 inch across, but more usually the quartz is late in 

 consolidation and embeds the other constituents. Analogy with 

 examples from both the north and south of the island, of less 

 doubtful origin, suggests that here is a portion of diabase which has- 

 been permeated and altered by an acid magma, and then included 

 as a fragment in the aplite. 



As a general conclusion, it is submitted that the mica of the 

 aplite-rock, 1 and therefore, by implication, that of the porphyritic 

 granite of the quarry, is of foreign origin ; and that it is a result 

 of the melting and incorporation of fragments of diabase during the 

 passage of the acid magma to its present position. 



One point of considerable interest remains. On the shore at the- 

 western end of the Bay the rapid change, from a coarse rock containing 

 a considerable quantity of quartz and closely related to the usual 

 aplite, into one in which quartz is conspicuously absent may be traced. 

 The disappearance of this constituent is so sudden that one may put 

 one's finger on the spot where it dies out. The quartz-bearing rock 

 contains mica, possibly some hornblende, now replaced to a great 

 extent by a green chlorite, and large red felspars, often f inch long, 

 with intergrown plagioclase. The quartz is found in grains '16 inch 

 or so across. Nor is free plagioclase wanting. The slide does not 

 differ in any essential point from those previously described. There 

 is on the whole more plagioclase in the quartzless rock, 2 sections 

 approximately normal to 010 indicating albite and oligoclase. Wedged 

 in between the felspars is some quantity of a soft black substance,, 

 which a thin section shows to be chlorite. It has a finely mottled 

 appearance in ordinary light, a speckled one between crossed nicols. 

 This substance, now represented by the chlorite, was clearly the last 

 constituent to consolidate, since it embeds the smaller felspars, 

 extends in tongues between the larger, and in places has split and 

 forced its way into them. With it are a little white mica and some 

 opacite. In addition to the fracturing there is evidence of some 

 corrosion ; but the felspars are not zoned. The rock is miarolitic. 



To give an explanation of its history is far from easy, and the 

 following one is put forward with some hesitation. In certain thin 

 sections from the neighbourhood the mica is seen to form flakes of 

 very irregular outline, embedding the crystals of felspar, and in 

 places enclosing them in a manner closely resembling that which 

 distinguishes the chlorite. Similarly, mica late in consolidation 

 characterizes many of the allied rocks from Southern Jersey. It is y 

 therefore, quite possible that the chlorite has replaced this mineral j 

 while the propinquity of mixed rocks arising from the alteration of 

 a dolerite by an acid magma strongly suggests that this mica is, in a 

 sense, of secondary origin. In the present instance the almost 

 entire absence of quartz in the chlorite-bearing specimen does not 



1 See the Ker. E. Hill & Prof. T. Gr. Bonney's paper on the ' Hornblende- 

 schists, Gneisses, etc. of Sark ' Quart. Journ. Geol. Soc. vol. xlviii (1892) p. 132. 



2 Two grains of quartz appear in one section, and therefore the term ' quartz- 

 less ' is not strictly applicable. 



