KiNAHAN — A Neio Reading of the Donegal Rocks. 17 



Detailed Descriptions. 



[These descriptions will refer principally to the rocks in the barony of Kilraacrenan, 

 as this district has been more especially studied by myself ; reference, however, will be 

 made to some of the rocks in the adjoining baronies.] 



For the purposes of this inquiry it seems expedient first to 

 give a general section of the rocks of the Older and Later Periods.^ 



There is a very continuous section across the Older Period 

 Rocks, in a nearly north-west and south-east line, from the valley 

 of Lough Beagh, past Lough Akibbon, to Grien Swilly, where 

 the rocks occur in the following order : — 



{A) Gneiss and foliated granite series, 

 {B) Grartan schist series — 



[a) Mica flag series. 



{h) Gartan Lakes group, including the limestone- 



diorite series, 

 (c) Sericite group. 

 {d) Yolcanic group. 

 {e) Cloncarn quartzite. 

 (/) Boheola group. 



{C) Gregory Hill series. 



[For General Section, see Plate I.) 



The rocks of the Gregory Hill series are apparently the 

 youngest group of the Older Period Rocks ; but they are separated 



rocks, called at that time Huronians. Since his visit to Canada, these "Western Onta- 

 rian rocks are better known, having been very minutely worked out, especially by Dr. 

 Lawson of the Canadian Survey, the results of the inquiry being that those rocks must 

 be newer than the Laurentians, but older than Logan's Huronians, they being the 

 eqmvalents of one of the groups now called Algonhian by the United States Geological 

 Survey. As pointed out by me in previous writings, if the markings in rocks in Glen- 

 dowan found by Mr. M 'Henry are graptolites, they suggest that such rocks may be 

 metamorphosed Cambrians, or even rocks of a later age. But we now learn that this 

 may not be so, as in the black shales of the Penokee Gogebig iron region [Huronians) 

 markings supposed to be organic have been found. If the latter supposition proves to be 

 correct, these Donegal rocks may possibly be pre-Cambrian, but are not of Laurentian 

 age. 



1 In the adjoining baronies of Eaphoe and Inishowen there are the previously 

 referred to rocks, evidently newer than those which are now to be mentioned ; these, 

 when properly worked out, may be found to belong to a stUl later period. They 



SCIEN. PROC. R.D.S. VOL. YII., PART I. C 



