Vol. 57.] 



AMOISTG THE MALVERN CAMBRIANS. 



171 



neighbourhood of the iron-ores, the grains of which it often fringes, 

 or in ferruginous serpentine. 



The ground mass, at any rate in the best-preserved rocks, is 

 holocrystalline, and usually has an appreciabl}' coarser texture than 

 that of the amphibole-bearing rocks previously described. In this 

 respect the groundmass approaches that of the more compact olivine- 

 diabases of the district, but never shows the ophitic structure invari- 

 ablyseenin the latter. In some cases (M 102,118,119,197,248,249, 

 & 268) flow-structure is seen, but ia the rest this structure is in- 

 conspicuous or absent. 

 Fig. 9. — A pseudomorph^ almost certainly The order of crys- 

 after olivine^ from an olivine-hasalt t all iz at ion has been; 

 (M236). x45 diam. apatite andiron-ores; 



olivine - phenocrysts ; 

 augite ; and finallyy 

 felspar. 



Among the secondary 

 products in the more 

 altered basalts (M 

 117^ 236, etc.) is a 

 colourless, sometimes 

 rather cloudy, mate- 

 rial which occurs in 

 patches both in the 

 groundmass and in the 

 serpentinized olivines. 

 It is frequently iso- 

 tropic or nearly so, 

 but at other times 

 exhibits fairly strong- 

 double refraction. The 

 patches often show a 

 polysynthetic struc- 



The centre is occupied by a large crystal of anal- 

 cinie (unshaded) embedded in calcite (dotted). The 

 pseudomorph is lined with confluent crystals of 

 analcime introduced from without. In the sur- 

 rounding groundmass the precise structure is not 

 clear, and the arrangement of plagioclase (lined), 

 serpentinous pseudomorphs after augite (outlined 

 inblack), and opacite (black) is semidiagrammatic. 



ture. Where distinct 

 crystals are seen these almost invariably give six-sided sections 

 (text-fig. 9, above). In one rock (M 236) such crystals are common 

 in spaces which, for the most part, and I think in all cases, mark 

 pseudomorphs after olivine. Where the crystals are large, they 

 often show under polarized light the peculiar division into fields 

 characteristic of analcime and garnet. The refractive index is 

 low, and in spite of the somewhat strong double-refraction (which,- 

 however, is not unparalleled) there can be no doubt that the 

 substance, in part at least, is secondary analcime. The 

 analcime tends to line the wall of the pseudomorph, but merges 

 gradually into the groundmass : sometimes one or more large crystals 

 have grown into the middle, where they lie partly surrounded by 

 calcite, or serpentine, or by both. The free faces of the small 

 crystals are usually parallel to the side of the attachment (text-fig. 9,. 

 above). The occurrence of analcime in a pseudomorph after olivine- 

 is not usual ; it has clearly been introduced from outside. 



