TEANSACTIONS OF SECTION C. 709 



the softei' rocks would then have been forced in between their ends. In this way 

 some of the thinner beds of quartzite have been conA'erted into lines of boulders, 

 ex. gr. at Ilowth. 



III. Ordovician. — The imconformity between this system and the Cambrian, 

 discovered by Jukes, is confirmed ; on the south side of the Cambrian masses of 

 Carrick the Ordovician slates are partly composed of fi-agments of green and grey 

 Cambrian slates. The distinction between the Cambrian and Ordovician rocks 

 as mapped by the Survey necessarily depends in most cases on ditierences of 

 colour, the Oi'dovician being usually, but not always, blackish in tint ; it is obvious, 

 therefore, that considerable room exists for error. These rocks are profoundly 

 modified on approaching the great granite mass which extends throughout the dis- 

 tricts ; the black slates and grits become lustrous with mica at a somewhat greater 

 distance from the granite than is indicated on the Survey maps, near Glendalough, 

 for at least half a mile farther from it. On approaching the granite closer, well 

 foliated andalusite-, garnet-, and mica-schists appear, the foliation corresponding 

 usually with tlie bedding planes, though these are folded upon themselves again 

 and again. Never, however, is a schist produced which by any possibility could 

 be mistaken for an Archisan rock. As in Wales so in this district the Ordovician 

 are distinguished by a profuse development of igneous rocks ; some of these are 

 contemporaneous, some intrusive, nearly all show signs of having been subjected to 

 o.xtreme pressure ; ash-beds are naturally converted into excellent slates, but flows 

 and dykes are usually also rendered schistose for a variable distance from their 

 margins, a central core remaining unaltered and thus aflording a means of dis- 

 tinguishing in the field between an ash and an originally solid couleg or dyke. 



I"\'. T/ic Granite. — Notwithstanding the somewhat positive assertions which 

 have been made as to the metamorphic nature of the granite, which extends for 

 a distance of sixty-five miles from north to south, and from eight to fifteen from 

 east to west, its intrusive character can be readily demonstrated ; not only is the 

 junction with the adjacent schists invariably well defined, without even the sugges- 

 tion of a passage between the two, but the granite frequently sends branching and 

 anastomosing veins into the surrounding rocks, and includes flarae-like fragments of 

 them ; the reason why a metamorphic origin has been so strenuously maintained 

 for this granite in particular is probably due to the fact that it shares the nature 

 of a scliist near the junction in so far that it possesses schistositj-, the planes of schis- 

 tosity in both the granite and the schist having the same direction ; it need scarcely 

 be added that this structure is the result in both cases of ' crush,' the abundant 

 slickeu-sided surfaces traversing the gneissose granite and its minute structure as 

 seen imder the microscope prove so much. Some granite which is not ajiparently 

 crushed also exhibits foliation, but of a different character, resulting from an 

 arrangement of the constituent mica in parallel planes ; these planes also are 

 parallel to the foliation of the adjacent schists; this would seem to indicate that 

 the pressure which folded the country was beginning to act before the gTanite had 

 every wliere solidified. Examples of this kind of foliation are exceedingly common 

 around the northern end of the granite district, but it dies out towards the interior. 



Y. Epoch (if Folding. — The folding of the Ordovician, as proved by the marked 

 discordance between its strike and that of the succeeding Upper-old-red Sandstone 

 or basal Carboniferous beds, took place before the Carboniferous system period. 

 The intrusion of the granite was post-Ordovician and pre-Carboniferous, and its 

 crushing and foliation occurred within the same interval. 



11. On Archcean Boclis. By G. H. Kinahan, M.R.I. A. 



In this communication is given a short description of the American (Dominions 

 and States) Archaean rocks, special attention being directed to the characteristics 

 insisted on by such American authorities as Dana, le Conte, Selwyn, &c., the most 

 important being the records, always found in America, of a vast lapse of time be- 

 tween the accumulation of the Archaean rocks and the subsequent deposition of the 

 later^rocks. The supposed Archseans in England are briefly referred to, and it is 



