﻿Vol. 6 1.] ARENIG FAWR AND MOEL LLYFNANT. 631 



certainly andesine ; and as the crystals are mostly elongate and 

 have a parallel or roughly-variolitic arrangement, the rock may 

 be termed an andesitic dolerite. The augite of the rock is 

 in somewhat smaller proportion than the felspar : it is a pale 

 yellowish or greenish variety, shows bright yellows between crossed- 

 nicols, and has the usual extinction about 34°. It, too, is frequently 

 quite idiomorphic, especially where fresh, but in other cases may 

 enclose the andesine ophitically. In addition to this, there is also 

 a small proportion of another, still more colourless, pyroxene, with 

 higher refractive index, which gives greys between crossed-nicols 

 and has almost straight extinction. This is probably a hypersthene 

 with a wide angle between its optic axes, and is extremely like 

 the hypersthene of the Tremadoc sills, as also that of the dolerites 

 of the Shelve country. Ilmenite, in networks of skeletal bars, and 

 apatite, in minute elongate needles, are also notable, as at Pen- 

 maenmawr. As alteration-products, chlorite of the form delessite, 

 calcite, and chalcedonic quartz are only too abundant, and pale 

 and dark epidotes occur (4665). The delessite is remarkable for the 

 way in which it ramifies between and around the best idiomorphic 

 felspar and augite- crystals, and during the weathering gives a sort 

 of pseudo-ophitic aspect to rock which was even once panidiomorphic. 

 Frequently it also forms good well-shaped pseudomorphs after the 

 elongate octagonal crystals of hypersthene. The quartz present 

 is nearly all chalcedonic ; and although I have sought for, I have 

 not been able to find, any micropoecilitic or eutectic intergrowths 

 between it and the felspar (4662-63). 



From the fact that the andesitic dolerites enclose pieces of 

 SJaimardia-Shale with undistorted fossils, and that the slightly- 

 hardened shales of that series which adjoin them are entirely un- 

 cleaved, one must conclude that the date of intrusion was prior to 

 the cleavage of the country. From the way in which the intrusive 

 sills seem to spread themselves in the neighbourhood of certain faults, 

 one might think that their intrusion was connected with the for- 

 mation of some of these. As will be seen from the foregoing descrip- 

 tions, the andesites and the andesitic dolerites appear to be closely 

 related ; and as the former, which are also the older, contain half- 

 digested fragments of the latter which are newer, both would seem 

 to have come from the same subterranean magma-basin. In this, 

 as in all other particulars, the andesitic dolerites closely resemble 

 the Shropshire dolerites which, as shown by Prof. Watts, 1 occasionally 

 cut strata of Llandovery or even Wenlock age, and I have every 

 reason to think that all are of the same general age. 



Other Intrusions [25] . 



Of other intrusions I need only mention the curious granular 

 andesites, which are so common at the horizon of the Niobe-~Beda, 

 and the hornblende-porphyrites, which are seen intrusive among the 

 Middle Lingula-Flags (Ffestiniog Beds) to the west of Blaen-lliw. 



1 Proc. Geo!. Assoc, vol. xiii (1894) pp. 339 et seqq. 



