Vol. 57.] 



AMONG THE MALVERN CAMBRIANS. 



163 



]. jtfectly fresh. Even in the best preserved examples, fibres of a sub- 

 stance with parallel extinction, indistinguishable from bastite, run 

 in between and parallel to those of tremolite, together with fibrous 



serpentine (presumably derived 



Fig. 3. — A -phenocryst of augite 

 from camptonite (M 209). 

 X 72 diam. 



-y&^^^ 



•^-. 



from the bastite) similarly orien- 

 tated. There is generally also some 

 structureless serpentine. In 

 most of the rocks the amphibole 

 is nearly or quite replaced by 

 bastite, serpentine, epidote, cal- 

 cite, and other secondary products. 

 Bastite is very generally re- 

 garded as derived only from 

 the rhombic pyroxenes. But 

 M. Boule ^ describes a case, 

 analogous to the present one, in 

 which all stages may be detected 

 in the transition from tremolite 

 to bastite in the serpentines of 

 the Allier. 



The augite-phenocrysts 

 (text-fig. 3) are usually far less abundant than those of amphibole. 

 They are of a very pale brown, show the usual form and cleavage, 

 and have an extinction-angle of about 42°. The substance is 



better preserved than that of the 



The phenocryst is traversed by ser- 

 pentine (dotted), possibly pseudo- 

 morphic after intergrown hornblende. 

 Oranules of opacite have collected 

 ground the augite, an unusual pheno- 

 menon in the Malvern intrusives. 



Fig. 4. — Phenocryst of am- 

 phibole from camptonite 

 (M209). X 72 diam. 



The above figure shows the re- 

 eorbed border and intergrown augite, 

 the eight portions of which (out- 

 lined in black) all extinguish to- 

 gether. Serpentine unshaded, faintly 

 outlined ; opacite (and leucoxene) 

 black. 



amphibole. They may usually be 

 distinguished at a glance from 

 crystals of the latter, even when 

 decomposed, by the absence of a 

 resorbed border. They are some- 

 times honeycombed (see text- 

 fig. 3) by spaces filled with ser- 

 pentine (or bastite) ; but I suspect 

 that in some of these cases the 

 sei'pentine represents intergrown 

 amphibole (see p. 164). 



The augite of the ground- 

 mass is similar to that of the 

 phenocrysts. It occurs in idio- 

 morphic grains and crystals similar 

 to, but less abundant than, those 

 in the olivine-basalts. A number 

 of granules are sometimes massed 

 together into little accumulations, 

 in the centre of which a serpen- 

 tinous remnant of some ferrc- 

 magnesian mineral is occasionally 

 seen (M 101a & 209). 



1 Bull. Soc. Geol. France, ser. 3, vol. xix (1891) p. 973. 



M 2 



