BY W. N. BENSON. 141 



with fresh augite, and chloritic decomposition-products of a 

 glassy base. 



The pyroxene, also, is generally fresh. Though in many cases 

 there has been a small amount of chlorite, uralite, epidote, and 

 calcite produced from the pyroxene, there is nothing that re- 

 sembles the highly altered state of pyroxenes in most British 

 spilites. No alkaline pyroxenes have been observed, nor any 

 rhombic pyroxene. Dr. Cox suggests that the quartz-dolerite of 

 the spilitic series in Wales(15) may be found to contain augite 

 rich in the rhombic pyroxene molecule. The present series of 

 rocks have been specially searched for this feature, and the optic 

 axial angle of many crystals was measured by the graphical 

 methods of Professor Becke. They proved to be very uniform 

 in character; the value of 2V lay in nearly all cases between 42° 

 and iS\ This is rather less than the normal value for augite, 

 but does not prove the presence of enstatite-augite, in which the 

 angle may diminish to zero. Calculation from the analyses shows, 

 howe^r, that some of the RSiOg molecule must be joined with 

 diopside molecule in the pyroxene developed. There is no 

 difference between the character of the pyroxene in a quartz- 

 dolerite and that of a dolerite free from quartz. Sahlite-structure 

 is never seen, though twinning is often present. The pyroxene 

 is generally greyish-green in colour when seen in section, and in 

 some of the more finely granular rocks it has a purplish tinge. 



The hornblende has the usual reddish-brown colour of horn- 

 blende in basic rocks, is rarely present, and in small quantities 

 only. 



The ilmenite occurs in plates of very irregular size and shape, 

 and, in more altered rocks, is frequently leucoxenised. The 

 apatite and pyrites are as usual; the mode of occurrence of the 

 quartz will be described below. 



The Texture. — The distinction between the dolerites and spilites 

 is one of grainsize only, and is a most indefinite one. There is a 

 wide range of texture between which every gradation can be 

 found. At the holocrystalline end of the series, the most marked 

 type of texture is that of the intersertal quartz-albite-dolerites of 

 Hanging Rock. In these, the crystallisation of the apatite 



