24 H. G. Sargent — The Penmaenmawr Intrusions. 



The greater part of the rock as seen in the field, including that on 

 the summit, is similar in all respects to the compact marginal rock of 

 Penmaenmawr, and, like the latter, often breaks with conchoidal 

 fracture. Round the base of the mountain, on the south and south- 

 east sides, the rock is slightly less compact and is precisely similar in 

 appearance to the transition-rock between the compact and fine-grained 

 varieties of Penmaenmawr. It may therefore be reasonably inferred 

 that only a limited amount of erosion has been effected by the stream 

 sweeping round the base. On Carregfawr also only surface rock is 

 available for examination, and it is all, including that of the summit, 

 of the same compact type as the marginal rock of Penmaenmawr. 

 Here, too, the platy divisional planes noticed above in the case of 

 Penmaenmawr are well developed. 



One description of thin sections will serve for the microscopic 

 structure of the two intrusions. The minerals present are felspar, 

 augite, and quartz, with apatite and iron-ores as accessories. 

 Bronzite appears to be entirely absent. Pelspar and augite occur 

 as phenocrysts. The rock is much decomposed and the felspars 

 are almost indeterminable. They appear, however, always to 

 extinguish straight or with low angles. Augite rarely presents 

 idiomorphic contours, but occurs mostly in the form of rounded 

 grains. Occasionally cross-sections occur showing good outlines, 

 the prism and pinacoid faces being equally developed. The fine 

 basal striation on the pinacoids, noted in the case of Penmaenmawr, 

 also occurs bere. The ground-mass consists of felspar-laths, but 

 more often of allotriomorphic felspar, and interstitial quartz. Some- 

 times the quartz occurs in large sheets, but never with crystal 

 outlines. Minute grains of augite are also present in the ground- 

 mass. Ilmenite appears to be the principal, if not the only iron-ore. 

 It occurs in minute grains and crystals of rectangular and hexagonal 

 outline. Generally speaking, the microscopic structure of these rocks 

 is very similar to that of the compact marginal rock of Penmaenmawr, 

 but quartz is on the whole more abundant here, and rhombic pyroxene 

 as noted above is absent. The structure of the ground-mass in places 

 may be termed sub-granophyric. 



The question of the name that should be applied to the rock of 

 which these three intrusions are composed appears to be a matter 

 of some difficulty. The writer would naturally defer to the opinion 

 of any one of the eminent geologists named above, of whom each has 

 in the case of Penmaenmawr bestowed upon it a designation of his 

 own. As it is clearly impossible to defer to them all, an independent 

 suggestion may be permitted. It seems desirable that, if possible, 

 the name adopted should connote not only internal structure but also 

 mode of occurrence. Seeing that we are dealing with masses of 

 hypabyssal habit, it will be well to discard any name properly 

 applicable to a rock of plutonic or extrusive origin. If this be 

 granted, we may at once rule out ' diorite ', ' norite ', and ' andesite '. 

 ' Diabase ' would doubtless be dismissed sans phrase by all British 

 geologists nowadays. 



In view of the determination of the felspars given above, and the 

 analyses quoted below, the name ' dolerite ' appears to imply a more 



