Marshall. — On Tridymite-TracUyte of LyttcUon. 373 



that the former is, if anything, the more vesicular of the 

 two. 



The mineralogical structure of this rock presents many 

 pecuharities, both in the nature of the minerals themselves 

 and in their association with one another. 



Tridj'mite occurs in considerable abundance, for, although 

 thirty-six sections of the rock were cut, some crystals of the 

 mineral occur in every section. The feldspars, both ortho- 

 clastic and plagioclastic, frequently occur in large porphyritic 

 crystals, and present the peculiar feature of a central core of 

 plagioclase surrounded with a mantle of orthoclase or sani- 

 dine. Ferro-magnesian constituents are rare, or entirely 

 absent, but magnetite occurs in considerable quantity, while 

 needles of apatite penetrate the ground-mass and feldspars. 

 As an accessory of somewhat doubt tul occurrence, zircon may 

 be mentioned. 



Tridymite, although often present in the ground-mass, is 

 generally found attached to the sides of vesicular spaces, 

 and sometimes completely fills the smaller vesicles. The 

 crystalline groups are, as a rule, of irregular shape, but 

 in some a fairly regular hexagonal boundary may be 

 observed. They are quite transparent, and possess a vitreous 

 lustre. 



In sections they appear generally as rounded aggregates, 

 quite clear and transparent, with numerous cracks that 

 resemble cleavage. With polarised light, however, they break 

 up into a number of irregularly-shaped areas of extremely 

 minute dimensions, but all possessing different optical orienta- 

 tion. To see the structure distinctly, a magnifyiug-power of 

 70 diameters or more should be employed, and it will 

 then be noticed that, although irregular, there is an approach 

 to the hexagonal boundary in the majority of the plates. 

 Each of these areas undoubtedly represents a distinct indivi- 

 dual, but, owing to then- extremely small dimensions, it was 

 found impossible to isolate any one of them and submit it to 

 optical examination w^itli the hope of forming any ideas as to 

 the system of crystallization. The peculiar irregular structure 

 of the aggregates is well shown by altering the focus of the 

 microscope by means of the fine adjustment, when it will be 

 seen that, even in the thinnest sections, there are several layers 

 of crystalline plates. 



The aggregates are frequently traversed by cracks which 

 seem to bear no definite relation to the outline of the indi- 

 vidual grains of the aggregates themselves. A peculiar feature 

 of many of these grains is a radial structure (shown in PI. 

 XLVIIL, fig. viii.). Although not universal, this structure 

 occurs in the majority of the grains. Figs. vii. and viii. were 

 both drawn from a section beneath a magnifying-power of 70. 



