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EEV. J. F. BLAKE 02^r THE 



sion with respect to the so-called " gneiss " or " dark schist." And 

 the conclusion to which I have been drawn, much against previous 

 prejudices, is, that it is all of igneous origin. And this I must 

 commence by demonstrating*. Now the mass to the north of 

 Holland Arms to a great extent lacks foliation, and is composed of 

 large crystals of green hornblende, mingled irregularly with dusty 

 felspar, in this respect agreeing with the less foliated masses at 

 Craig -yr-allor. In the hill to the north of Y-graig there is a 

 parallel arrangement of the more hornblendic and the more 

 felspathic portions, producing a leuticularly banded rock, again to 

 be matched at Craig-yr-allor and other localities in the central 

 district. These hornblendes pass, in places, at their edges into a 

 bright-blue tinted mineral, and the felspar into epidote. At the 

 edge of this mass, on the south side of Y-graig, the hornblende, 

 partially converted into chlorite, appears in isolated needles, mingled 

 with numerous grains of epidote, the minute portion intervening 

 being quite clear, and possibly quartz or felspar. This mass is 

 clearly isolated in the midst of the grey gneiss to be presently 

 described. To the east are two other apparently isolated patches 

 at, and to the north of, the Gaerwen windmill. In one we have 

 the leuticularly banded diorite, and in the other the same network 

 of hornblende-needles and epidote grains, in part foliated and in 

 part not. IS'ow rocks of this type, for the most part beautifully and 

 finely foliated, with exquisite contortions, pass into the glaucophane- 

 rock which I have lately described t- A map of the distribution of 

 these rocks amongst the mica-schists shows that they are mostly 

 sporadic ; so that though it might be possible to conceive of such an 

 arrangement by complicated folding in more than one direction, 

 the far more natural explanation is that of their being intrusive 

 masses, especially as there is, within certain limits, considerable 

 variation in their respective neighbours. Again, though some of 

 these patches are foliated, others are not, and therein they agree 

 with the rocks at Gaerwen, as they do in all other respects. These 

 observations alone might suggest the igneous origin of the rock, but 

 fortunately in the two cuttings made by the railway we get 

 decisive proof. In that near Llangaffo, which I examined in the 

 company of Prof. Sollas, we have the following section (see fig. 14). 

 Entering from the east, we find the micaceous grey gneiss slightly 

 dipping E., and it soon becomes contorted. This contortion is 

 seen to be in connexion with a dark foliated rock, which intrudes 

 upon it in an irregular manner. This is followed by another 

 smaller intrusion, and in all the neighbourhood of these the gneiss 

 is much disturbed ; its folia are lost in immediate contact with the 

 intrusion, and it looks almost like a halleflinta, but is still gneissose 

 in structure. Purther west these phenomena disappear; the grey 



* Prof. Bonney, indeed, calls one example submitted to him a diorite 

 (Geol. Mag, 1880, p. 127) ; and Dr. Callaway has lately shown that the foliated 

 rock at Gaerwen is of the same character (Brit. Assoc. Rep. 1887). Further than 

 this no one seems to have considered the possibility of an igneous origin for 

 these schists. 



t Geol. Mag. dec, 3, vol. v. p. 125. 



