Dr. C. Callaioay — Notes on Mefamorplmm. 219 



mainly of two kinds. A metamorphic rock associated with unaltered 

 beds was proved to be of the same age as the latter, because either 

 it overlay or passed into them. Thus, as an illustration of the former, 

 the newer gneissic rocks of the Northern Highlands were said to 

 be Ordovician, because they rested on Ordovician limestone and 

 quartzite. Murchison saw the gneiss lying on the top of the lime- 

 stone or the quartzite, and he said that the upper beds were the 

 younger. Everybody believed him, except Nicol, who died discredited 

 and disheartened. Now, we know that Nicol was right, and the 

 great authority, misled by superficial appearances, was wrong. This 

 line of evidence, however, I do not for the present follow; but 

 confine myself to the latter fallacy. I propose to examine a few of 

 the cases in which a passage has been or might be alleged to exist 

 between metamorphic masses and contiguous unaltered strata. 



Perhaps I cannot commence better than by exposing one of my 

 own mistakes. In an early stage (1879) of my investigation into 

 the metamorphic rocks, I examined similar sections at Twt Hill, 

 near Caernarvon, and Nebo, near Amlwch, Anglesey ; and, under 

 the influence of the popular teaching, I believed there was in both 

 cases a passage between gneissic rock and conglomerate. Professor 

 Hughes, supported by Dr. Eoberts, subsequently attacked this view, 

 and my own observation has canvinced me that they are right. At 

 Twt Hill, the arkose, being composed of the same constituents as 

 the adjacent granitoid rock, looked so much like the latter that I 

 was deceived, and I followed my predecessors into the error of the 

 old school. This mistake carried with it the section at Nebo. 

 Believing by analogy that the Nebo conglomerate was Archfean, I 

 was bound to make the overlying shales unconformable ; and 

 the very singular way in which the shales are thrust on to the 

 conglomerate and squeezed into its hollows, gave a colour to the 

 interpretation. However, having been once bitten, I hope I shall 

 be twice shy. 



Other examples in Anglesey will be given in a paper now in the 

 hands of the Secretary of the Geological Society. I discussed an 

 alleged case of progressive metamorphism in the district south of 

 Wexford in this Magazine, November, 1881. 



Alleged Passage hettoeen the Llanberis Slates and the Metamorphic 

 Bocks of Anglesey. 



A good example, on a large scale, of the fallacy which I am 

 discussing is furnished by the reasoning by which Sir A. C. liamsay 

 attempts to prove the Cambrian age of the green schists west of the 

 Menai Straits. The Llanberis series becomes, he says, more altered 

 towards the west, first, by the " porphyry " of Llyn Padarn, and 

 then by the granitoid mass north of Caernarvon ; or, to represent 

 the view which he seems to prefer, these crystalline rocks are them- 

 selves portions of Cambrian and Silurian formations melted down. 

 I will not stop to discuss the possibility of the conversion of slate and 

 conglomerate into felsite, to which the chemist might raise some ob- 

 jections. It is sufficient to point out that the conglomerates on Llyn 



