214 



that the felspar plays the role of ground-mass. A somewhat similar rock occurs 

 in Swinnerton Park, eight miles N.E. of Stafford. This rock, however, 

 contains less felspar and is richer in olivine, much of which is perfectly 

 unaltered. It forms, according to Mr. ALLPORT, a part of a mass of 

 dolerite of somewhat variable texture. 



If we take a general view of the dolerites of the Midland Counties 

 we note that they are characterized by the presence of olivine, often rich 

 in iron, and by the frequent occurrence of deeply-coloured augites. In 

 both these respects they differ markedly from the dolerites of the North 

 of England, and resemble some of those occurring in the Midland Valley 

 of Scotland and in the Island of Arran. The normal rock of Pouk Hill 

 is substantially identical with many Tertiary dolerites occurring in the 

 north of Ireland, west of Scotland and Iceland. 



North Wales. The counties of Merionethshire and Caernarvonshire 

 contain immense masses of igneous material. The contemporaneous 

 volcanic rocks of Merionethshire form a large portion of the mountain 

 ranges of Cader Idris, the Arans, the Arenigs, the Manods and 

 Moelwyn. They consist for the most part of acid material and belong 

 to the Arenig period. The corresponding rocks of Caernarvonshire form 

 a large portion of the Snowdonian group of mountains and belong to a 

 later period, viz., the Middle Bala of SEDGWICK. In both areas we find 

 numerous exposures of basic igneous rocks. These are. however, for 

 the most part intrusive and not interbedded. The most important 

 exposure of contemporaneous volcanic products of basic composition 

 occurs in the neighbourhood of Ehobell Fawr, five or six miles N N.E. 

 of Dolgelli. Here we find basic tuffs and breccias containing well-preserved 

 zonal augites, zonal plagioclase, brown hornblende, lapilli, and a matrix 

 which doubtless represents the finer fragmental volcanic material. The 

 alteration to which these rocks have been subjected has given rise to the 

 development of chlorite, epidote and other alteration products. These 

 rocks are associated with typical ophitic diabases. The massive igneous 

 rocks occasionally, though rarely, have a basaltic rather than a doleritic 

 structure, and so far as petrographical characters are concerned these 

 varieties may well be contemporaneous lava flows. Professor RAMSAY 

 however, states W that " in no single case in Merionethshire is there any 

 line of greenstone that can be proved to have flowed across a land 

 surface or sea bottom. There may be some difficult cases, and there 

 are many in which a cursory examination would recognize no difference 

 between their mode of occurrence and that of the felspathic lavas, but a 

 thorough acquaintance with the igneous rocks of Merionethshire will 

 show that in no single instance is it safe to include any line of greenstone 

 in the list of lava currents, and most of them may be safely dismissed 

 from that category." 



There can be no doubt, therefore, that in the vast majority of cases 

 the greenstones are intrusive. They occur as sheets running more or less 

 parallel with the bedding, and sometimes reaching a thickness of several 



(1) The Geology of North Wales. Survey Memoirs, Vol. III., 2nd Edit., 1881, p. 99. 



