1884.] Microscopic Structure of some Bocks from Ecuador. 433 



of dull reddish-gray not very vesicular scoria, probably lithologically 

 in close alliance with (1) and (2). 



The following is a description of the microscopic structure of 

 No. (4) . In the earlier stage of consolidation are (a) felspar crystals, 

 probably in great part labradorite. The enclosures are frequently 

 variable in nature, quantity, and arrangement. Sometimes their dis- 

 position is zonal and external, sometimes it is central. Among these 

 enclosures are pale green belonites (? hornblende), colourless belonites, 

 pieces of brown glass, often abundant, containing gas cavities and 

 crystallites of magnetite, cavities containing bubbles, which occupy 

 one-sixth or one-seventh of the whole space. The exteriors of the 

 crystals are frequently broken-looking or corroded, (b) A pyroxenic 

 constituent, of which some is certainly augite, but a part (the smaller) 

 probably hypersthene. The former rather frequently contains enclo- 

 sures ; among them are magnetite and grains of a slightly irregular 

 oval outline, sometimes nearly 0'003 inch diameter, occasionally asso- 

 ciated with gas cavities (? felspar), (c) Grains of iron oxide, pro- 

 bably magnetite. The part of later consolidation is a pale brownish 

 glass, speckled with opacite and crowded with acicular microliths, six 

 or seven times as long as broad, which generally do not exceed O001 inch 

 long. These are colourless and probably to a large extent felspar. 

 No. (5) differs from the last rock but little in its microscopic structure; 

 it has a rather clearer ground-mass and perhaps not quite so many 

 granules of black iron oxide. The crystals of felspar are similar, but 

 there is also a large number of well-formed lath-shaped crystals, 

 measuring in longer diameter above 0*01 inch. Two varieties of 

 augite, a greenish and a brownish, are present, together with a little 

 of the greenish mineral which, as it has an orthorhombic extinction, I 

 refer to hypersthene. Microscopic examination of ($) shows it to be 

 not materially different from (4), except for the presence of the more 

 decomposed spots, mentioned above. The glassy base is perhaps 

 a shade more colourless. Both augite and hypersthene are present. 

 No. (9.) In the first stage of consolidation we have rather numerous 

 felspar crystals, with the usual variable enclosures — glass cavities 

 with fixed bubbles, microliths, nearly all of which exhibit the charac- 

 teristic twinning of plagioclase, though one or two show Carlsbad 

 twinning and may be orthoclase. The former usually extinguish at 

 moderately large angles, ranging from rather less than 10° to more 

 than 20° with the twin-plane. In one, where the twinning is sharply 

 defined, the extinctions are 21° and 30° respectively on either side of 

 the twin-plane. It is therefore probable that these crystals are neither 

 albite nor oligoclase. The pyroxenic constituent appears, as above, to 

 be of more than one kind. The most abundant is a brownish, rather 

 dichroic mineral, black bordered, and sometimes rather " dirty," 

 owing to inclusions. In colour and general aspect it more resembles 



