Marshall. — On Tridymite-Trachyte of Lyttelton. 375 



drawn in the annexed figures. They do not call for any 

 special description. 



In order to show clearly that the supposed sanidine is not 

 plagioclastic feldspar cut in the direction of a plane parallel to 

 the brachypinacoid oc P oc (010), PL XLVIL, fig. v., may be 

 mentioned. In the large crystal drawn in this section the 

 traces of the faces which appear longest have cleavage-cracks 

 parallel to them, and must therefore be the traces of the base 

 O P (001) and the chnopinacoid oc ?- cc (010). 



As these traces are almost exactly at right-angles to one 

 another, and extinction takes place when the cross-wires are 

 parallel to these traces, the section must be cut in the zone of 

 the orthopinacoid and base. If, now, the mineral were plagio- 

 clase, twinning parallel to the brachypinacoid (corresponding 

 to the chnopinacoid of sanidine, as shown below) would be ob- 

 served in such a section — that is, supposing the usual isomor- 

 phic relations to hold good, as they will be shown to do when 

 the plagioclases are considered. The fact that the mineral 

 extinguishes parallel would in itself generally be considered 

 sufficient to show that it is sanidine and not plagioclase. The 

 truncations of the angles are, of course, due to the develop- 

 ment of a clinodome. The same method of reasoning may be 

 applied to fig. vi., PL XLVIL, where there is a core of plagio- 

 clase surrounded by a broad rim of sanidine. This example is 

 even more conclusive than the last, as here the plagioclase 

 is twinned on the albite type, and it cannot therefore be 

 pleaded that the mineral is untwinned plagioclase. Since 

 sanidine is doubtless present in this crystal, it may be said 

 with safety that the other unstriated feldspar possessing similar 

 interference colours is sanidine. 



Plagioclase is also present in crystals with idiomorphic 

 outlines, and generally of a far larger size than the crystals of 

 sanidine. Almost invariably, however, there is an investing 

 mantle of sanidine, which sometimes is of far larger diameter 

 than the core of plagioclase, but in other cases far smaller. 

 In general, the plagioclase can hardly be called idiomorphic, 

 as it passes in many cases gradually into the surrounding 

 sanidine, the exact boundary-line being hard to determine. 

 Occasionally, as in PL XLVL, fig. iv., there appears to be an 

 outer zone of plagioclase possessing different optical orientation 

 from the inner one. Cleavage-cracks can seldom be seen, and 

 zonal structure is rare and inconspicuous when developed. 



Twinning is splendidly developed, according to three well- 

 defined laws — Carlsbad, albite, and pericliue. Tlie Carlsbad 

 twins are exceedingly common, the two halves being frequently 

 as sharply defined as in sanidine. The investment of sanidine, 

 in every observed case but one, is also twinned, but its orienta- 

 tion is different from that of the plagioclase, although the 



