NEIGHBOURHOOD OF SARN, CAERNARYONSHIRE. 461 



plates of considerable extent, and in the absence or rarity of ilmenite 

 in the dolerites. In all these and other characters the rocks in 

 question have close affinity with the dolerites of other parts of 

 Caernarvonshire and Anglesey. Similar dykes on the shores of the 

 Menai Straits cut through Carboniferous strata * ; and others in the 

 Anglesey coal-field are demonstrably post-Carboniferous and pre- 

 Permian t. Such evidence as is obtainable points, then, to the con- 

 clusion that the dolerite-dykes of the Sarn district, to which we 

 may add some of those which intersect the neighbouring " green 

 schists " J:, were injected in the interval between the deposition of 

 the Carboniferous and the Permian formations. 



Waiving this point, however, and excluding the dykes and minor 

 eruptive masses from consideration, we find that the district of Sarn, 

 probably during the age in which the Bala rocks were laid down, 

 was the theatre of igneous activity on a large scale. The strata 

 were invaded by eruptive magmas of very different chemical consti- 

 tutions, and rocks of widely diverse characters have resulted from 

 the consolidation of those magmas ; so that we find, speaking broadly, 

 acidic rocks occupying the northern and western portions of the 

 district, intermediate or sub-basic forming a broad band across the 

 middle, and basic and ultra-basic developed in the south. These 

 latter, the heavier materials, appear to have spread in the form of 

 laccolites, possibly at a considerable depth ; the magma which was 

 the origin of the granite may, on the other hand, have been in con- 

 nexion with some of the extrusive or volcanic outflows which are so 

 striking a feature of the Bala stage in Caernarvonshire. 



The isolated intrusions of gabbro cannot safely be correlated with 

 other rocks in the area : their passage into hornblendic rocks, and 

 the evident relation of that change to the coming in of the schistose 

 character, though points of novelty in North Wales, are closely 

 paralleled in many other districts with which geologists have been, 

 made familiar. 



\_Note. — The specimens and slides illustrating this paper are in the 

 "Woodwardian Museum at Cambridge.] 



Discussion. 



Mr. Teall would have been glad to have heard more of the 

 details of the paper before attempting to criticize it. As a point of 

 nomenclature it would be satisfactory to know if we should have 

 different names for rocks containing primary or secondary horn- 

 blende. In the mass of diabase and picrite there seemed to be a 

 differentiation and coming in of parallel structures ; in this direc- 

 tion we may hope for a solution of the problem of the origin of 

 gneisses. 



* Geol. Mag. dec. 3, toI. iv. p. 409 (1887). 



t Ramsay, loc. cit. p. 264. 



\ The igneous mass south-west of Aberdaron, terminating in the headland of 

 Pen-y-cil, also has two generations of felspars, but differs in other respects from 

 the dolerite of the dykes in question. 



