160 J. A. PHILLIPS ON THE SO-CALLED 



rock projecting above the surface on the eastern side of the valley 

 of Tolcarn the majority of the large crystals of felspar are tolerably 

 perfect, and, when seen in polarized light, exhibit very distinctly 

 the coloured strise usually considered to be characteristic of plagio- 

 clase ; on the other hand, the augite has been entirely replaced 

 either by hornblende or viridite, while magnetite and apatite remain 

 unchanged. The material constituting the numerous veins by 

 which this rock is traversed is exactly similar to that forming the 

 matrix, excepting that it contains a little more green hornblende. 



The augite, in specimens broken from the rock near the north- 

 western foundations of the new parsonage, is, on the contrary, but 

 slightly altered, while the felspar is much decomposed, the crystals 

 of this mineral having frequently become almost merged in a 

 colourless slightly opaque base, in which the form of the original 

 felspar crystals may still be traced. The rock from this locality 

 contains, disseminated through it, a considerable amount of brown 

 hornblende, viridite, magnetite, a little mica, and apatite, with 

 felspar, and pseudomorphs after felspar interpenetrated by green 

 belonites. 



The compact greenstone exposed in a quarry on the opposite side 

 of the road has been subjected to a much larger amount of alteration 

 than either of the foregoing ; there are, however, to be observed 

 outlines of felspar crystals, some portions of which exhibit striae 

 when seen in polarized light ; and pseudomorphs after augite are 

 tolerably distinct. The apatite is unchanged ; but many crystals of 

 magnetite have been, to a great extent, replaced by a grey siliceous 

 mineral *, 



The somewhat slaty rock which comes in immediate contact with 

 the above consists of a translucent cloudy base, through which 

 hornblende and magnetite are thickly disseminated, together with a 

 little, apatite. Although its original structure is, to a very great 

 extent, obliterated, faintly delineated outlines both of augite and 

 felspar can be detected. 



At Battery Point there is a rock mainly consisting of tolerably 



* In the first specimen in which these pseudomorphs were observed the whole 

 of the magnetite had been replaced. I was therefore at a loss to determine 

 the nature of the original crystals, and submitted a section for examination to Mr. 

 Allport, who returned it without having been able to arrive at a decision. A 

 few days after the return of this section, I received a note from Mr. Allport, 

 in which he stated that he had then prepared a specimen containing pseudo- 

 morphic bodies, enclosing unaltered magnetite, and that he was satisfied they 

 were pseudomorphs after that mineral, adding, " I have no doubt they will be 

 found to be siderite." Subsequent examination of a large number of such 

 patches in various stages of alteration leaves no doubt of their being pseudo- 

 morphs after magnetite ; but the substance by which that mineral has been 

 replaced is not generally siderite. On heating a thin section of one of these 

 rocks with hydrochloric acid in a hollow cell before the microscope, no evolution 

 of carbonic anhydride took place ; and no action upon the pseudomorphs was 

 apparent after boiling for twenty minutes in dilute hydrochloric acid; the 

 whole of the apatite, however, was removed by this treatment, leaving numerous 

 hexagonal holes through the section. 



