F. P. Mennell — Wood's Point Dyhe, Victoria, Australia. 395 



be a chloritic or serpentinous decomposition product, but from its 

 relation to the other minerals this seems very unlikely, and it may 

 be looked upon with some certainty as devitritying glassy material. 



The above description applies to the leading (and most basic) type, 

 which may be classed as a hornblende porphyrite, the term porphyrite 

 being applied in the sense of a hypabyssal equivalent of the diorites. 

 A few varieties, apparently coarser-grained in the hand-specimen, 

 show, curiously enough, considerably more crypto-crystalline ground- 

 mass, while in the finer-grained varieties the hornblende is much 

 more frequently idioraorphio and takes on forms similar to the 

 basaltic variety. One fine-grained marginal modification is very 

 acid in character, being of a grey colour in hand-specimens and 

 sliowing minute cavities. The silica, kindly determined for me by 

 Mr. L. Ludlow, reaches the high figure of 77-4 per cent. The 

 chief feature of interest is the occurrence of cordierite, distinguished 

 by its peculiar twinning and its biaxial interference figure in 

 convergent polarized light. Most of the granules show some 

 approach to crystal outline and are twinned, the interference tints 

 of the individuals being dilierent in cases where the extinctions are 

 simultaneous. One crystal, though not quite perfect, nevertheless 

 well illustrates the multiple twinning of the mineral (see Fig. 2). 

 Jt diff"ers from the form figured by Rosenbusch ^ in the fact that the 

 twin planes bisect the sides, not the angles, and the boundaries 

 of the individuals are somewhat irregular, while an extra one is 

 wedgred in between two of the othei's. 



Fig. 2. — Cordierite Felsite. Crossed nicols, x 50 diam. 



Osann has accounted for the presence of primary cordierite in 

 a somewhat similar rock as due to the complete absorption of 

 fragments of cordierite gneiss, resulting in the local formation of 

 a magma containing an abnormal percentage of alumina, from which 

 cordierite crystallized out in due course. In this case we have no 



1 " Mikroskopische Pliysiographie," pi. xviii, fig. 1. 



