^74 PROF. G. A. J. COLE ON THE AGE OF CERTAIIS" [May I 899, 



Geological Survo}- Map, Sheet 34. While we conld detect in tho 

 field no alteration in the soft red sandstones, yet the furious inter- 

 mingling of granits and sedimentary rock seemed fair evidence of 

 intrusion. 



The specimens collected were examined microscopically ; but no 

 evidence of metamorphism was even then procurable from the 

 sandstone. This is a fine-grained reddish-brown rock, consisting of 

 angular quartz-grains and abundant pale mica : the mica is bent 

 against the quartz and is clearly detrital. The tufted patches of 

 muscovite that also occur probably represent altered potash-felspar, 

 which was converted into muscovite before it was washed into tho 

 sandstone. The felspar of the adjacent granite is frequently in this 

 condition. One or two broken prisms of tourmaline, and clear 

 grains of triclinic felspar, also lie in the thin sections of the sand- 

 stone. The opaque cement is limonitic. 



There is no material in this Devonian sandstone which could not 

 be derived from the underlying granite ; the grains, moreover, are 

 but little waterworn. The granite, both at the contact and in the 

 grit-like mrxss exposed above Aghnagreggan tuck-mill, is finer in 

 grain than much of the local granite ; that of the new exposure at 

 Einmore, for instance, in Mullaghslin Glebe, 2 miles to the west, 

 is far more typical and coarse. This fact may show that we are 

 near the surface of cooling of the original granite-dome, but it is 

 not in itself evidence of intrusion. The Aghnagreggan granite^ 

 moreover, shows many signs of alteration. The felspars are full of 

 minute tuftv muscovite ; and rusty brown granules, which I regard 

 as ferriferous epidote, abound ihroughout them. Similar granules 

 occur in the felspar of the curious granite of Limehill Upper, north 

 of Pomeroy, which is rich in calcite ; and in the aplite of Carndaisy 

 Glen, on Slieve Gallion, where the brown particles have arisen, 

 together with rutile, freely in the altered biotite, and less freely 

 in the felspars. Calcite occurs also in this latter rock, and 

 lirae may have been imported from without. Whatever its nature, 

 the ferruginous brown alteration-product in the Aghnagreggan 

 granite would impart ample colouring-matter to any sediment 

 formed in the vicinity. The granite at the tuck-mill is thus stained 

 and tlecked with orange, the decaying felspars giving it a further 

 resemblance to the adjacent sandstone-series. 



It became necessary, then, to revisit this critical section, and to 

 obtain a larger exposure alons: the bank of the little Granagh Burn, 

 in which it had been originally unearthed. The result is to prove 

 the derivation of the sandstone in large part from the granite, and 

 to destroy the theory of intrusion of the granite into the sandstone. 

 Along a slightly varying level in the sandstones, irregular lumj s 

 of granite, sometimes bounded \)y joints, sometimes subangular, are 

 found abundantly, and form more than half the bulk of certain 

 specimens brought away. Large quartz-grains, and flecks of granite 

 partially broken up, occur in the groundwork of this junction-rock'. 

 Vertically below it, larger masses of granite come in, which may 

 be only great boulders on the pre-Devonian land-surface. Enough 

 is seen, however, to show that here the granite occurs priactically in 



