Revieivs — A. Harlcer's Bala Volcanic Scries. 229 



prize, lias published the substance of his essay, illustrated by three 

 small scale and three large scale maps. 



The introduction divides the county into three areas ; two lie east 

 and west respectively of the Llyn Padarn ridge, while the other 

 includes the Lleyn peninsula. A bibliography is appended. 



There are five main outflows of rhyolitio lavas, whose limits are 

 traced on a map which enables the author to suggest that the 

 intrusive masses near Y Foe! Fras and that of Mynydd Mawr are 

 their centres of eruption. A few, but too few, analyses of the 

 rliyolites are given, and these indicate the presence of the follow- 

 ing minerals, in order of abundance — quartz, orthoclase, albite, 

 pinite, and magnetite. The usual structures are crypto-crystalline 

 and micro-crystalline, but a remarkable type from the lowest lavas 

 is of the nature of an ophitic structure, in which quartz encloses 

 felspars ; something similar to this was observed by Teall 1 in the rock 

 of Penmaenmawr. Fragmental and quasi-fragmental rocks seem to 

 be much less common than is usually supposed. In a chapter on 

 the nodular felsites Mr. Harker comes to the general conclusion that 

 the structures arise from solid spherulites, in which either the centre 

 has been replaced by amorphous silica or the shrinkage due to 

 molecular re-arrangement has formed concentric cracks along which 

 alteration has proceeded. 



The acid irruptive masses are usually augite granophyres, such 

 as those of Y Foel Fras, and granite porphyries generally augitic, 

 like the intrusions of the Rivals, and occur as cylindrical necks or 

 laccolites. Hornblende is often absent from these rocks ; the well- 

 known dark patches are of concretionary origin. 



In the description of the intermediate rocks that from Penmaen- 

 mawr receives still another name, " bronzite bearing quartz dolerite," 

 and the augite and bronzite andesites of the Lleyn are investigated; 

 most of the latter appear to be intrusive, though conclusive evidence 

 is not given, while there are certainly some andesites associated 

 with agglomerates and ashes. 



The adoption of the word " sill " for the intrusive diabase sheets is 

 devoutly to be wished for; such sills are known to be very com- 

 monly intruded between the beds of the other volcanic rocks, and, 

 with the doubtful eruption of Porth-Dinlleyn, the diabases are 

 never in the form of lava-flows. The absence of dykes is signi- 

 ficant, and the few exceptions appear to be later intrusions injected into 

 the post-Carboniferous joint system. The sills have probably been 

 intruded into cavities or planes of weakness formed during the folding 

 of the strata. Olivine is absent even from the post-Carboniferous 

 intrusive rocks. 



• After a chapter devoted to the altered gabbros, picrites and 

 hornblende diabases of the Lleyn, the author gives a general review 

 of vnlcanicity in Caernarvonshire. A map elucidates the distri- 

 bution of the cleavage-planes, and makes it quite clear by the 

 increase of contortion and the perfection of cleavage that the 

 pre-Cambrian mass of felsite near Llyn Padarn had a large share 

 1 Teall, British Petrography, p. 273. 



