OE THE DIMETIAN AND PEBIDIAN ROCKS. 165 



augite or olivine are more distinct ; that it also belongs to the 

 basalt group is without doubt. 



4. Consists principally of quartz with some felsitic material. In 

 thin section quartz is seen to be the prevailing mineral ; it presents 

 no traces of crystals, but exists only as a granular mass, the felsitic 

 material, which is much decomposed, filling the interstices and spaces 

 between the quartz grains. Occasionally, though rarely, a crystal 

 may be observed in the felsitic substance presenting traces of the 

 banding of a plagioclase ; chlorite is present. 



5. Is apparently a breccia, consists almost entirely of subangular 

 and rounded grains of quartz with remains of felspar crystals. Some 

 of the fragments are evidently derived from a rock like No. 4. The 

 whole is cemented together by a dark grey material, appearing like 

 dust under the microscope. 



Pebidian. 



1. The rock constituting the fragmentary part of the agglomerate 

 appears to consists chiefly of a spherulitic felsite porphyry, fre- 

 quently banded with alternately dark greenish-grey and light 

 pinkish-grey bands. It presents a very fine-grained texture, with 

 a dull hornstone-like fracture, and exhibits, microscopically, distri- 

 buted crystals of felspar having a feeble lustre. It weathers cream- 

 white, the spherulitic structure when present being then perfectly 

 exhibited on the surface. On freshly fractured surfaces, however, 

 it is not to be detected, even with the aid of a lens, and the very 

 large spherules, so prominent in the weathered specimens, present 

 but faint indications of their presence. 



Seen in thin sections under the microscope, however, the struc- 

 ture is at once recognizable, the whole mass appearing to consist of 

 spherules, frequently arranged in well-marked bands of varying 

 dimensions, and also confusedly grouped without any apparent ar- 

 rangement. Felspar crystals of apparently two kinds, orthoclase 

 and a plagioclase, are porphyritically developed — the former indicated 

 by its characteristic Carlsbad twins, and the latter by traces of its 

 original fine striation, though both are much altered, probably by 

 kaolinization. Quartz grains, or rather immature crystals, are pre- 

 sent but not in quantity, and are characterized by the absence of 

 glass inclusions, those of fluid being but sparingly contained in them. 



Around these felspar and quartz crystals the bands of spherulites 

 stream, meeting on both sides, and in some cases being developed 

 upon the crystals themselves, forming a botryoidal crust. In the 

 larger spherulites, crystals and groups of crystals of felspar are fre- 

 quent. The small interspaces between the spherules are mostly 

 filled with a granular dark opaque substance, quite undefinable 

 even with high powers, while occasionally a micro- crystalline 

 felsitic mass with minute grains of quartz is substituted. A bright- 

 green chloritic mineral is also sometimes present, probably derived 

 from grains or crystals of augite, the outlines of some crystals of 

 which, though very ragged, are still very distinct. The dark-green 

 wavy bands seen in hand specimens differ from the pinkish-grey 



