374 Transactions. — Geology. 



Fig. vii. shows the normal structure of triclymite with polarised 

 light, except that the different areas are not shaded as they 

 appear beneath the microscope. The interference colours are 

 always low, but it was found impossible to give the general 

 effect by shading the different areas. 



FeldsiKirs. — These are perhaps the most interesting of all 

 the minerals in this rock, for the sections serve to show that 

 the isomorphism of orthoclase and the more acidic plagioclases 

 (andesine) is almost exact — so nearly so that crystallization of 

 sanidine can proceed with as great energy round a core of 

 plagioclase as round one of sanidine. 



Sanidine occurs in beautiful glassy porj)liyritic idiomorphic 

 crystals, as well as in granular aggregates with irregular out- 

 lines. The porphyritic crystals are generally of small dimen- 

 sions, and often give square sections, which show^s that the 

 crystals are not generally elongated in the direction of the 

 chief axis. Cleavage is plainly seen in most of the sections, 

 and a faint zonal structure can generally be observed. Be- 

 tween crossed nicols the interference colours are low, greys 

 and bluish-grey of a low order being general, and affording a 

 marked contrast with the brilliant interference colours of 

 plagioclase in the same section. 



Twinning is exceedingly common, and, though well-de- 

 veloped examples of the Carlsbad type are frequent, the ma- 

 jority of the twins are irregularly penetrating, frequently 

 without the slightest indication of any regular law ; but at 

 other times twins resembling those formed on the Baveno and 

 Manebacher law have been observed. 



The Carlsbad twins — composition plane oc ? oc (010), twin- 

 ning plane cc ^ oc (100) — are numerous, occurring in crystals 

 of all sizes, even the microlites in the base sometimes showing 

 this type of structure. Some examples of these twins are 

 given in the figures. 



A twinned crystal that bears some resemblance to a Baveno 

 twin is shown in PI. XLVIIL, fig. ix. Unfortunately, this 

 crystal seems to have been considerably corroded before the 

 rock cooled, and the margin has therefore become somewhat 

 rounded. Supposing this to be a Baveno twin, it would con- 

 sist of four individuals such as those figured in Dana's " Text- 

 book of Mineralogy" (page 100, fig. 325, and page 325, fig. 587). 

 The composition and twinning-plane are the clinodome 2 5 cc 

 (021). Although the traces of the twinning-planes are not 

 exact diagonals, they do not show more irregularity than 

 many of those of the Carlsbad twins. A very similar crystal 

 has been found in another section ; while in a third there is 

 another, consisting of two individuals, the twinning trace being 

 more nearly a diagonal. 



Irregular penetrating twins are frequent, and some are 



