Yol. 57.] 



AMONG THE MALVEEX CAMBEIANS. 



165 



often of a reddish colour. They commonly give tabular sections, 

 and not infrequently show zonal growth. They are simple, twinned 

 once or several times. Rarely both albite- and pericline-lamellation 

 are seen. Well-preserved crystals are not sufficiently numerous 

 to permit of a determination of the angle of extinction, but 



angle observed 

 Fig. 6. — Semidiagrammatic figure 



of a phenocryst from camptonite 



(M209). X 72diam. 



The above figure shows augite intimately 

 penetrated by fibrous serpentine, probably 

 pseudomorphic after tremolite. [The 

 fibrous character of the serpentine is ex- 

 aggerated in the figure.] The different 

 poi'tions of augite (outHned in black) ex- 

 tinguish practically together. There is no 

 resorbed border. The felspar-microlites 

 show a flow - arrangement around the 

 whole. See also fig. 7, p. 167. 



the largest 



was 44° ; others give quite 

 small extinction-angles on 

 the two sides of a plane of 

 twinning. Probably more 

 than one kind of felspar is 

 present. Pseudomorphs con- 

 taining calcite, serpentine, 

 and opacite are not un- 

 common. 



The felspars of the 

 groundmass are frequently 

 reddish, and usually show lath- 

 shaped sections. The extinc- 

 tion of the laths is usually 

 undulatory ; but they appear 

 to belong for the most part to 

 a basic andesine. They 

 often show a tendency to split 

 up into thin fibres. 



The ilmcnite - plates, 

 which are of any size, are 

 usually very ragged, and 

 rarely show any trace of the 

 six-sided form. This is seen, 

 however, much more fre- 

 quentl}'^ in the small crystals 



of the groundmass, where 

 they are mingled with octahedra of titaniferous magnetite, 

 and with irregular grains of one or both minerals. 



Apatite is present, both in the form of long needles and short 

 prisms. 



It is not easy to see where, in a classificatory system, the 

 amphibole-bearing rocks described in the foregoing pages should be 

 placed. By some petrologists they w^ould doubtless be termed 

 porphyrites or andesitic amphibole-basalts ; but, as lam 

 unacquainted with any basalts or other rocks agreeing precisely 

 with them, I have thought it best to place them among the 

 lamprophyres as andesitic camptouites, or as camptonites 

 of the Malvern type. 



