KiNAHAN — A New Reading of the Donegal Rocks. 15 



and quartzites of that country all belonged to one and the same 

 formation, and that their lithological differences were solely due 

 to the different amounts of metamorphism that each of them 

 had undergone. Logan seems to have been the first to point out 

 that this was not so; while more recently Irvine, Lawson, and 

 others have unravelled the geology of the country north-west, 

 west, and south of Lake Superior, proving that the great series of 

 rocks, formerly classed together, are divisible into groups belong- 

 ing to distinct periods of time, although in some cases the rocks in 

 one group may be scarcely distinguishable, lithologically, from 

 those in another. 



In the different American reports, but especially in the admir- 

 able memoir by the late Dr. Irvine (" Classification of the Early 

 Cambrian and pre-Cambrian Formations," by E,. D. Irvine),, 

 some of the descriptions, maps, and sections are very illustrative 

 of what may be found in the county Donegal. Of these we may 

 mention specially the description, map, and section of the country 

 in the district of Marquette, Michigan, as there the proofs of the 

 distinct ages of the rocks are so similar to what may be obtained 

 in North Donegal that one description will do for both.^ 



The Older Period Rocks were upheaved, contorted, and invaded 

 by granite — (a) after which they were considerably altered, and 

 then invaded by another granite ; {b) and afterwards denuded 

 before the Later Period Rocks were deposited on them. 



The post-metamorphic granite veins (&), although frequent in 

 the Older Period Rocks, never extend up into the Later Period 

 Rocks.' 



Furthermore, connected with the Older Period Rocks there 

 are dykes and other intrusions of metamorphosed traps. These 



^ It is very interesting to me to learn that the conclusions as to the Donegal rocks, 

 suggested by the late G. A. Kinahan, and confirmed by myself, should, as above pointed 

 out, be additionally confirmed by Irvine's recently published maps, sections, and reports 

 of the Lake Superior district ; more especially as I know of my own researches that in 

 America, as ia Donegal, the difference between the quartzites of the diSerent formations 

 is so slight that ordinary geologists could not detect it. 



^ In the north portion of the county of Donegal there was a still later intrusion of 

 granitic rocks. This must have taken place at a comparatively recent time, as the 

 courses of elvan and felstone that branch off from the granite intrudes are very 

 little broken or otherwise disturbed : they running continuously, irrespective of the 

 upthrusting and other vicissitudes to which the country rocks were subjected. 



