280 A. OEIKIE ON THE SUPPOSED 



so that they must be measured at right angles to the line taken by 

 Dr. Hicks. I am not aware of any method by which we can 

 measure the thickness of a square mile of granite. 



To the phenomena of contact-metamorphosis I shall return in the 

 second part of this paper (p. 317). The evidence now brought forward 

 is, I submit, amply sufficient to prove that, whether studied in hand- 

 specimens, in microscopic slices, or in the numerous natural sections 

 which show its geotectonic relations, 'the " Dimetian " group of Dr. 

 Hicks, instead of being a ridge of Pre-Cambrian metamorphic rock, 

 is really a boss of eruptive granite, later in date than the Cambrian 

 strata through which it has been intruded, and that the term 

 " Dimetian ", so far at least as regards its original locality, must be 

 abandoned. 



2. " Akvonian." 



The rocks grouped under this name by Dr. Hicks at St. David's are 

 thus described by him. 



" The rocks now included in this group I originally associated 

 with the Dimetian ; but in the year 1878 * I separated them from 

 the latter, under the above name. 



" On the Survey Maps they are coloured generally as felstones 

 and porphyries, usually intrusive amongst Cambrian or Lower Silurian 

 rocks. They consist in reality of flows of rhyolitic lavas, alternating 

 with felsitic breccias and halleflintas. The strike is from JN". to S., 

 and, hence, discordant to those newer rocks with which they are 

 usually surrounded, as also to the underlying Dimetians. Like the 

 Dimetian, this is a highly acid group, being mainly made up of the 

 types of rocks known as the quartzo-felspathic. But', instead of 

 being like these, chiefly of clastic origin, we have here a great series 

 of acid lavas mixed up with a comparatively small proportion only 

 of rocks of a clastic nature. In colour these lavas vary from being 

 very dark (almost black) to a light grey, and from deep red or violet 

 to flesh-colour. The flow-structure is usually well marked, and in 

 many cases 1 he spherulitic structure also. A large number are por- 

 phyritic, from the minute crystals of felspar or quartz. The halle- 

 flintas are more siliceous-looking than the rhyolites, and have a 

 horny-looking texture and fracture. Under the microscope they are 

 still more easily distinguishable. Their chief peculiarity, perhaps, 

 consists in the manner in which some of the quartz becomes separated 

 away into nests, so as to give the rock a curious pseudo-porphyritic 

 appearance ; whilst the intervening parts exhibit the appearance of 

 a micro-crystalline mass of quartz grains, with intervening felsite. 

 The breccias usually consist of fragments of lavas and halleflintas, 

 like those in association with them, and the pieces angular. 



"This group, therefore, is characterized by being for the most 

 part made up of acid lavas, breccias, and compact siliceous rocks of 

 the halleflinta type, and as usually having the strike in a direction 

 from N. to S."t. 



* [Quart. Journ. Geol. Soc. vol. xxxv. p. 285 (1879).] 

 t Proc. Geol. Assoc, vol. vii. no. i. p. 62 (1881). 



