28 THE VOYAGE OF H.M.S. CHALLENGER 



place of the primary plagioclase. The sections of augite show the fragmentary nature 

 of the mineral even better than those of felspar by their notched and cracked outlines. 

 This pyroxene is almost colourless, as was the case also in the other rocks from this 

 locality which have been described. It is recognised by its vivid colours of polarisation 

 and by its characteristic cleavage. Although epidote appears for the most part to 

 have been formed where it is found, there are sections of it which bear unmistake- 

 able traces of having belonged to an original crystalline rock. We have said that 

 epidote has crystallised in the vesicles of the diabases from St. Thomas, where it 

 assumes the form of almost colourless fibro-radial groups, more or less spherical or 

 ellipsoidal in shape ; now, in the specimen just described fragments of this amygdaloidal 

 epidote are found. This mineral is characterised by its brilliant polarisation colours, 

 its pale tint in ordinary light, absorption, and the citron yellow pleochroism it 

 exhibits, as well as by a slightly rough appearance of the surface of the sections. Like 

 the felspar, this epidote appears to show vague polarisation phenomena, resulting 

 from the stress to which the rock was subjected. Aggregations of epidote, chlorite, 

 and quartz, which are sometimes seen as yellowish green or almost colourless 

 patches, may very well be derived from the decomposition of a bisilicate, all further 

 traces of which have vanished through alteration. Besides zeolites resulting from the 

 transformation of part of the felspathic substance, these secondary minerals are found 

 in all the vesicles of the rock, where they appear as a coating or as small colourless 

 crystals sometimes prismatic. Occasionally they all appear to be chabasite, twinned 

 crystals of which are recognisable. 



To summarise briefly the leading features characterising the descriptions given 

 above, we may say that, taking account of all the transitions which have been 

 shown, the specimens from St. Thomas represent an uninterrupted series, from 

 amphibolic rocks with acid plagioclase (oligoclase) to augitic rocks containing a plagio- 

 clase approaching anorthite. 



