HARD WICKE'S SCIENCE-GOSSIP. 



THE ROCKS OF THE NORTH-WEST OF IRELAND. 



By CHAS. WARDINGLEY. 



O study the geology 

 of the north-west of 

 Ireland is practi- 

 cally to study on a 

 somewhat extensive 

 scale the entire sub- 

 ject of regional me- 

 tamorphism. Only 

 three very limited 

 areas in the counties 

 of Donegal and 

 Londondeny have 

 types of other than 

 ancient altered 

 rocks. In the ex- 

 treme west, with 

 Dunglow as a 

 centre, granite is 

 the prevailing mate- 

 rial. On the south- 

 east shore of Lough Foyle the carboniferous limestone 

 becomes visible, while in the south-west of Donegal 

 a section occurs about twenty-eight miles long, from 

 Blue Stacks Hills to Lough Melvin— through Donegal 

 and Ballinbra — in which the old red sandstone and 

 carboniferous rocks are seen dipping to the south off 

 the older mica slates which flank the 2200 feet of 

 granite forming the Blue Stacks. With the exception 

 of these three areas, the rocks of the countiy under 

 consideration — the North-West Highlands of Ireland 

 — may be referred to the Lower Silurian, or, to use 

 the more modern term, Ordovician Period. 



I think it was Dr. Hull who first drew attention to 

 the extremely interesting and curious geological fact 

 that if a straight line be drawn on a stratigraphical 

 map from Galway Bay, on the west of Ireland, to 

 the Firth of Tay, on the east of Scotland, it would 

 divide the older palaeozoic rocks of the British Isles 

 into two sharply defined series. North and west of 

 No. 344. — August 1893. 



this line all the Silurian and Cambrian rocks have 

 been altered and their organisms obliterated, while 

 south and east of the same lice the members of the 

 same series are in a great measure unchanged and 

 their fossils, graptolites, trilobites, and brachiopods, 

 preserved. 



Our district belongs to the first-named series, and 

 accordingly any one who has hitherto only been 

 acquainted with Salop or North Wales outcrops 

 must be prepared for a great difference both in the 

 mineralogical composition of the rocks and in their 

 general appearance. In fact so different are they in 

 every respect from typical Llandeilo and Caradoc 

 beds that in themselves they give absolutely no clue 

 to their identity, and we can only arrive at their 

 geological horizon by comparing them with other 

 recognised altered strata, and by observing the 

 position they occupy relative to other rocks. It has 

 been definitely ascertained that they lie unconform- 

 ably upon Cambrian rocks, and that their general 

 inclination is in a north-westerly direction. In 

 an unaltered state the Ordovician system presents 

 to us a series of shallow-water formed sandstones, 

 and deeper-water formed flagstones and shales. In 

 places where these are most fully developed they 

 seem to merge gradually into, or alternate with, each 

 other, the result, probably, of differences in local 

 geographical conditions. In Co. Londonderry the 

 common rock is mica slate, or schist, passing from 

 that into clay slate, chlorite slate, gneiss, and 

 quartzite. This slate is made up of minute particles 

 of mica and quartz, arranged in parallel layers, and 

 splitting quite easily along the lines of lamination. 

 The result is that the mica is more observable than 

 the grains of quartz, which are only evident on the 

 cross fracture. In many places in Scotland where 

 similar metamorphosed rocks are found, garnets are 

 of frequent occurrence, but in the north-west of 

 Ireland they seem to be somewhat rare, an examina- 



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