ON A HILLSIDE IN DONEGAL : A GLIMPSE 

 INTO THE GREAT EARTH-CALDRONS 



By GRENVILLE A. J. COLE, F.G.S. 



Professor of Geology in the Royal College of Science for Ireland 



The days for great sweeping surveys of the earth have long 

 been over ; and the condottieri of the geological armies, who 

 would ride rough-shod across half a continent, lived to find 

 local toilers rise against them, and to see weapons forged in 

 nooks and crannies raised to bar their future passage. For a 

 time, indeed, geological observation threatened to become lost 

 in detail. The description of new fossil species seemed to go 

 forward without regard to zonal grouping, or to the relations 

 of faunas as a whole ; while the neat labelling of a microscopic 

 section became the logical end of an unnatural system of petro- 

 graphy. The signs of the times, after this diversion towards 

 the histology of the subject, again indicate a desire to survey 

 and correlate ; and modern generalisation is certain to be wisely 

 tempered by the advice of the specialists who now abound in 

 every field. 



Just as a lake may nurture a peculiar variety of trout, so 

 some corner of the earth may have its own mineral associations ; 

 but many geological phenomena recur over such wide areas 

 that we may fairly regard them as fundamental. Hence, when 

 we sit down, for our personal edification, upon a bare and 

 glaciated boss in Donegal, we may extend our view from the 

 quartzite of Aghla and the granite of Glendowan to the 

 crystalline crust at large, the field of many a tourney. 



The rock around us seems gnarled and twisted ; it is 

 composed of interfolded sheets of differing constitution, some 

 of which contain abundant dark and gleaming mica, while 

 others are light in colour and rich in quartz and felspar. 

 The rock is, in fact, a "gneiss," not merely with a general 

 parallelism among its constituents, but with a well-marked 

 banded structure. Here and there a sharply bounded band 

 of quartz appears, with mica-schist on either side ; the surfaces 



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