96 Ernest W. Sheds: 



bleached eystals. Several sections of another iiiiiieral are present. 

 It occurs as colourless prismatic sections, showing minute fluid and 

 other inclusions, with high refractive index and low polarization 

 colours which are grey to yellow, of the first order. Two cleavages 

 are noticed, a well-defined one parallel to the longer axis c)f the 

 crj'stal, and a less well developed one at right angles to this. 

 A few sections showed straight extinction, but the majority 

 extinguished in an oblique position. Th0 nlaximum extinction 

 angle observed was 43 deg. from the longer axis. It is invariably 

 associated with a marginal colourless mineral of lower refractive 

 index and higher polarization colours. This mineral extends 

 inwards from the margins of the crystals, and appears to be an 

 alteration product consisting of a white uniaxial micaceous 

 mineral. Most of the sections of the mineral show the emer- 

 gence of an optic axis in a somewhat oblique position. The 

 mineral is andalusite. The .sections are too thin to cxhiliit 

 the characteristic pleochroism, and the high angle of extinction 

 noticed in some sections is to be connected Avith the large optical 

 axial angle exhibited by this mineral. ■'■ 



The more altered type of slate (Section 507) shows complete 

 recrystallization of the clastic materials. The rock consists 

 mainly of a number of interlocking qvuirtz granules and mica- 

 ceous minerals. The latter include biotite. muscovite and 

 bleached biotite (.'). No trace of a spotted structure is seen, 

 but the original bedding planes are defined by lines along which 

 there is a greater concentration of biotite and hematite, and 

 larger crystals of the micas occur along these lamin*. Among 

 the minor constituents minute rutiles occur, and a few iileudii-dic 

 granules of tourmaline,* which have been included in the 

 bleached micas. Andalusite is not represented in this rock. 



These altered rocks, containing an abundance of micas, arc evi- 

 dently rich in alkalies. It is therefore probable that in the 

 formation of the shale the alkali contents were not leached out 

 as sometimes happens. It must be remembered, however, that 

 the alkali contents of the shales may have boon reinforced by 

 thermal solutions passing out from the margin of the gra.nitic 

 intrusion. 



1 The miiiilie.'.'s of tlie rock sections refer to tlie riiiversitv colleetioii of rocU slices. 



