John Parkinson — Gabhro of Pegli, N. Italy. 297 



epidote, with perhaps occasionally zoisite, the character of the altered 

 groundmass changes. White mica is occasionally met with in some 

 quantitjr, at times constituting, in small closely felted flakes, nearly 

 all the groundmass of the rock ; at others including manj' of the 

 colourless grains. Here and there the alteration, has given rise to 

 a brownish-black mineral, far from clear, but showing occasionally 

 very roughly hexagonal and prismatic outlines, which from the 

 extinction is probably monoclinic. No definite cleavage can be 

 observed, and under a fairly high power the structure appears to be 

 finely granular. This may be a somewhat unusual form of epidote. 

 Characteristically it occurs in groups of close-packed grains, about 

 •1 mm. across, often pear-shaped in outline ; but in many cases the 

 dimensions of the individuals scarcely become apparent till the nicols 

 are crossed. The polarization colours are low, but the impure 

 character of the mineral may have something to do with this. An 

 earlier stage in the life-historj^ of this mineral is found in the in- 

 definite clouds and patches of an opaque substance lying round and 

 indistinguishable from the more definite grains. As a rule crossing 

 the nicols discloses the presence of obscure individuals, but here and 

 there even these are hard to discern ; there is merel}'^ an even dis- 

 semination of blackish material with very little double refraction. 

 In one or two slides long prismatic or acicular crystals (average 

 size about •12 mm, by •01 6 mm.) are arranged in twos and threes 

 in diverging groups ; and locally are rather abundant. Those 

 I think may possibly be actinolite. Their opaque appearance, due 

 apparently to the amount of impurities contained, as well as their 

 small size, renders an exact determination impossible. They are 

 irregularly terminated, rather fibrous in structure, with a greenish 

 tinge, and with weak double refraction. They are set as a rule in 

 a closely felted mass of white mica, so that here, as in several other 

 places, the more acid or felspathic element usually produced during 

 the decomposition of one of the more basic felspars seems absent. 



One or two accessory constituents remain to be noticed. In a few 

 slides apatite occurs in some quantity, either in well-formed idio- 

 morphic crystals — cross sections sometimes measure •TS mm, across — 

 or in groups of irregular grains. Such allotriomorphic outlines as 

 these grains possess, together with a rather dusty appearance which 

 the mineral has, were unlike the usual habit of apatite ; and I am 

 indebted to Mr. J, J. H. Teall, F.E.S., who very kindly examined 

 the slide and made the determination for me, suggesting at the same 

 time the usual molybdate test as a ready means of settling the point 

 definitely. This was done, and since the usual yellow precipitate 

 followed there can be no doubt that the mineral is really apatite. 

 Pyrites is present in some quantity, sometimes showing idiomorphic 

 outlines, and there is in addition a considerable quantity of ilmenite 

 altered in greater or less degree to leucoxene. The sp, gr. of the 

 gabbro as determined by a Walker's balance is 3-129, This is the 

 mean of determinations made on several specimens, the results 

 agreeing closely with one another. The sp. gr. of the more felspathic 

 variety is 2-977. 



