Vol. 62.] IGNEOUS AND SEDIMENTARY ROCKS OF LLANGYNOG-. 239 



The vesicles are filled with a soft mineral, which appears darker 

 than the surrounding rock. These highly-vesicular flows have a 

 mean specific gravity of 2 - 70. 



Under the microscope the rocks are seen to be vesicular hyalopilitic 

 andesites, with soda-lime felspar abundantly developed and occur- 

 ring in two generations. The felspar of the first generation builds 

 lath-shaped crystals, unzoned, but usually twinned according to 

 the Carlsbad and albite-laws. Pericline-t winning is not common, 

 but interpenetration-twins and stellate groups are of frequent 

 occurrence. The felspars are, as a rule, fairly fresh; but occasionally 

 they show a development of secondary quartz, pale-green chlorite, 

 and dusty material of a micaceous character. The series of low 

 symmetrical extinctions on the twin lamellae, and on sections at 

 right-angles to the acute bisectrix, would indicate that the majority 

 of these felspars lie in composition between oligoclase and andesine. 

 The second generation of felspars consists essentially of microliths 

 with low or almost straight extinction, indicative of oligoclase, 

 arranged in bunches and fan-like aggregates which make up the 

 bulk of the groundmass of the rock, and cause it to present almost 

 a variolitic appearance. 



No original ferromagnesian mineral in the unaltered state now 

 exists in these rocks, but undoubted pseudomorphs after augite 

 occur plentifully, showing the external form in many cases (see 

 PI. XXV, fig. 1). The interior of these pseudomorphs is composed 

 of a mosaic of secondary quartz and chlorite. Felspar-laths may 

 be seen to penetrate the augite, and we therefore infer that the 

 ferromagnesian mineral was of a later consolidation than the first 

 generation of felspars. 



The vesicles, which range up to '2 inch in diameter, are either 

 spherical or irregular in shape, and are filled with fibrous chlorite- 

 minerals in radiating masses, either alone or with a little calcite 

 or secondary quartz. Many of the larger vesicles are lined with a 

 thin layer of secondary quartz, -02 to *04 mm. thick. The ground- 

 mass, although largely composed of felspar-microliths, contained a 

 small quantity of interstitial glass, now more or less completely 

 devitrified and clouded with iron-ores. The chlorites filling the 

 vesicles are of two varieties : the commonest is pale-green, feebly 

 pleochroic, building fibrous and (less frequently) scaly aggregates. 

 The zone of elongation of the fibres has a positive sign, and the 

 mineral has a low double refraction (y — a = -005). 



The other mineral, which is sometimes intergrown with the 

 former, has a deeper colour and stronger pleochroism, but occurs in 

 fibrous aggregates. The maximum absorption is for light vibrating 

 parallel to the length of the fibres, of which the zone of elongation is 

 positive. For light vibrating parallel to the long axis ot\the fibres, 

 the absorption gives a yellowish-brown, and for a direction at 

 right-angles thereto yellowish-green. In a slide a little more than 

 •03 mm. thick, the mineral gives the yellow and red polarization- 

 tints of the first order, which would indicate a birefringence of 

 from "014 to -015. From the foregoing considerations, we are led 

 to regard the mineral as one of those to which the name delessite 



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