Kinahan — On Granite and Metamorphic Rocks. 231 



[The older rocks {Arenig (?) and Cambrian (?) ), after their deposition, were distorted, 

 contorted, thrustcd, altered, and invaded by granite and other intrusive rocks, and then 

 denuded, before the newer series of rocks were deposited on them. This seems to be 

 satisfactorily proved ; for although the later rocks were subsequently greatly disturbed, 

 displaced, and even inverted in some localities, yet there are still ocular proofs that one 

 series lies unconformably on the other, as at a little north of the Malin Coastguard 

 Station ; while in places the basal beds of the later series contain thrustcd fragments 

 of the granite, gneiss, &c. ; thus proving the metamorphism and denudation of the older 

 rocks prior to the accumulation of the others. Afterwards, however, the later rocks, 

 and probably the older ones, were subjected to another period of metamorphic action.] 



In the rocks (Ordovician and Llandovery (?) ) above the uncon- 

 formability, there are also many intrudes and sheets of whinstone 

 (Donegalyte), and a few intrudes of felstone ; while in places, 

 associated with the Donegalyte, are small and large masses of 

 agglomerate and tuffs ; the most extensive recorded being in the 

 neighbourhood of Croaghan Fanad. The lower portion of the later 

 rocks is usually partially altered, while the upper series (Kilgarvan 

 group) is generally not so. If the latter are altered, it is, for 

 the most part, in isolated detached portions. In the Kilgarvan 

 group are some well-marked, apparently interbedded, sheets of 

 Donegalyte. 



As a general rule, felstones and allied rocks are not common 

 in the Co. Donegal ; still in the northern and north-western parts 

 there are many courses, dykes, and patches of elvan, porphyry, 

 and felstone occurring, as adjuncts of the intrusion of the later 

 granite. 



The later granite occur in rocks that seem to belong to the 

 older stratified formations (Cambrian (?) and Arenig (?)) ; but that 

 they were intruded later than the accumulation of the younger 

 formations (Ordovician and Llandovery) is proved by dykes of the 

 porphyries and felstones from them being found in the latter. 

 There are also in a few places intrudes of these granitic rocks in 

 what may be outlying tracts of the later rocks. 



In the western part of the county, especially, there are dykes 

 of small dimensions, sometimes very numerous, of either melaphyre 

 or doleryte, which are considered by M'Henry to be of the same 

 age as the Tertiary rocks of the Co. Antrim. 



Among these different rocks there are some which are eminently 

 suitable for building purposes, and for cut- stone and polished 

 work ; but on account of their out-of-the-way position, very little 



