Reviews — Prof. RulVs Physical Geology of Ireland. 123 



The geological structure of Ireland, on the -whole, is singularly simple. Over 

 more than nine-tenths of the total area we find no rocks of a date later than Palae- 

 ozoic (Upper Carboniferous, with a trace of Lower Permian, if these be Palaeozoic) 

 until we come to the more recent Tertiary, post-Pliocene, or drift deposits, as the 

 author calls them, that is, we have no representatives of any but the oldest and the 

 newest groups of rocks ; while even in the remaining small area in the north-east 

 of Ireland, although some of the Mesozoic rocks are represented, there is still an 

 immense period (including all the time represented in the closely adjoining counties 

 of England, by the wide-spread, and as regards both the agricultural wealth and the 

 loveliest scenery of the country, all-important groups of the Lias, except the lowest 

 beds, the entire series of the Oolites, the "Wealden Purbeck. and the Lower Cretaceous 

 rocks) which is in Ireland without a single record. All the evidence, also, would 

 seem to point to the fact, that this absence of the rocks is due to an original absence 

 of deposition, and not to any subsequent denudation ; and would, therefore, indicate 

 that for the vast lapse of ages, represented by all the rocks between the Carboniferous 

 period and the commencement of the post-Tertiary, the greater part of Ireland was 

 elevated into dry land, while the sea covered widely the whole central portion of 

 England. The little tabular view (given on p. 3) of the different formations on 

 both sides of the Channel, showing which are present and which are absent, will be 

 found very usefril. 



Briefly referring to the Cambrian rocks, so well known to visitors of Dublin, from 

 their readily accessible positions at Bray Head and Howth, to the south and north of 

 Dublin Bay, the author proceeds to the Lower Silurian beds, which rest unconform- 

 ably on these older rocks, the gap between the two representing the absence in 

 Ireland of the Lingula Fhgs, or Upper Cambrian of North Wales (Enghsh Survey 

 classification). These Lower Silurian deposits are among the most interesting to 

 the student of Irish geology, from the fact that they occur in two very distinct 

 phases, now perfectly established as representative one of the other. In the one 

 they are still unaltered, and still full of fossils, though indurated, contorted, and dis- 

 turbed; in the other they have become highly crystalline, metamorphic schists, 

 gneiss, hornblende schists, etc. The age of these rocks is well fixed both by position 

 and by fossils, as belonging to the great Lower Silurian, and being probably about 

 that of the Caradoc and Llandeilo groups. Beds of limestone occur, though rarely, 

 and contain fossils representative of the Bala beds. These rocks, in their meta- 

 morphic condition, occupy four areas in the north and north-west of Ireland. 

 1. "West Galway and part of Mayo. 2. North of Clew Bay. 3. The Ox Mountains 

 from Lough Cong to Lough Gill. 4. Donegal and Derry. To the east of the 

 island there is a north-eastern area in the highlands flanking the Mourne Mountains, 

 and north of Dundalk, and a south-eastern extending from near Dublin to the coast 

 of the county of Waterford. 



There are only two areas of any great extent of Upper Silurian rocks in Ireland 

 — one near the Killaries, in Galway, between Lough Mask and the Atlantic. And 

 this is of high interest, as affording undoubted evidence of the date of the metamor- 

 phism of the rocks which we have just noticed — evidence which the country of 

 Scotland, so rich in metamorphic phenomena, has not yet afforded. There the 

 rocks immediately resting on the metamorphic Lower Silurian beds are the Old Red 

 Sandstone, and it is therefore only possible to assert that the metamorphism took 

 place prior to the formation of the Old Red Sandstone. But here— in Galway — 

 resting on metamorphosed rocks of unquestionably the same general age as those 

 of Scotland, we have, quite unconformably, masses of conglomerates, grits, shales, 

 etc., formed out of the debris and waste of these metamorphic rocks, and which 

 themselves, though containing beds of volcanic ash, and sheets of felspathic lava, 

 — evidence of contemporaneous volcanic action during their deposition, — still exhibit 

 no trace of that wide-spread and deep-seated metamorphic action so strongly developed 

 in the lower beds. These beds in Galway probably represent the Wenlook and 

 Ludlow beds of England. The only other important locality is near Dingle, where 

 a very large series, probably representative of the whole Upper Silurian series of 

 England, occur. These have derived some special interest from the occurrence above 

 them, and conformable to them, of the series to which Jukes gave the name of 

 ' Dingle beds,' a great thickness of grits, shales, and conglomerates, which are them- 

 selves unconformably covered by the lowest beds of the Old Red Sandstone. 



The absence in Ireland of any marked representative of the true Devonian, or 

 marine equivalent of the Old Red Sandstone, is noted, and the distinctively Irish 



