1 86 The Irish Nahtralist. 



except those called "true unconformability" by Indne, seems 

 to be the reason wh}^ this observer has in his report (pp. 390 

 et seq.), taken such pains to explain and illustrate the different 

 types of unconformability. This treatise of his might most 

 advantageously be studied and adopted by some British geolo- 

 gists, as pointed out in m}'' paper, "A New Reading of the 

 Donegal Rocks.'" Some of Irvine's and Van Hise's descrip- 

 tions of "cases in which the overlying strata are folded," 

 might have been written to describe sections in Donegal. 



The Donegal rocks have been very fully explained in my 

 paper just now referred to. It is therefore only now neces- 

 sary to give short general descriptions, and refer to some of 

 the errors in the since-published Explanatory Memoir of 

 Sheets 3, etc. The editors of this memoir state that my 

 opinions are not agreed to by any of the other surveyors. 

 This, in part, is incorrect, as at least one of the surveyors did 

 agree with me. 



The quartzytes of Donegal are more jumbled up than those 

 elsewhere in Ireland. In this county and the adjoining por- 

 tions of Derry, Tyrone, and Fermanagh, there are, at least, 

 three distinct terranes. The upper, as in previous writings 

 suggested, is probably the equivalent of the slate series (Upper 

 Ordovicians) of Munster; the irregular subordinate fine con- 

 glomerates, like the muUaghsawnytes,^ being very charac- 

 teristic of this series. 3 The middle terrane I have suggested 

 to be probably the equivalent of the lower Ordovician (Cara- 

 doc sandstone and lylandeilo). At its base there are massive 

 quartz3^tes: the rocks on which so much difference of opinion 

 has arisen. Evidently the rocks of the lower terrane (Kil- 

 macrenans) had received rough treatment prior to those of 

 the second terrane having been deposited on and against 

 them, as they had been invaded by intrudes of granytes and 

 other intrusive rocks; also they had been distorted by up- 

 thrusts and other convulsions, while subsequently they had 

 been considerably denuded. That the rocks of the middle ter- 

 rane must be more recent than those of the lower, is ocularly 

 demonstrated, as the younger rocks lie across the foldings in 

 the older, as is conspicuously illustrated in the escarpment 

 between Eough Finn and Glenties. 



At Knockanteenbeg^ the section "shows the necessary irre- 

 gularity in the strati graphical distribution of a basal con- 

 glomerate at the junction of two discordant formations. "^ 



1 Sci. Pro. R. Dublin Soc. 1890. 



^ Mullaghsawnyte is a fine conglomeratic rock, characteristic of the 

 upper terrane in Donegal, and largely developed in the ridge of MuUagh- 

 sawn, whence the name, invented by Mr. F. W. Bgan. 



3 "Geolog}'" and "Economic Geology of Ireland." 



* A precis of the eliminated description of the rocks of the Knockan- 

 teenbeg and Gartan districts, also the transverse and vertical sections, 

 will be found in the paper, ^" A New Reading of the Donegal Rocks." 



5 Irvine's "Memoir," p. 398, and Diagram Fig. 73. 



