STRUCTURE OF SOME ANGLESEY ROCKS. 



237 



Considering the locality and the consequent age of these specimons, 

 their structure is most interesting. So far as my experience goes, 

 there is nothing in either which would have awakened my sus- 

 picions as to their age, had they been labelled as from some locality 

 where Tertiary or even more recent trachytes are found. 



EXPLANATION OF PLATE VIII. 



Fig. 1. Map of Anglesey, showing the distribution of the Palaeozoic, Pebidian ; 

 and Gneissic series. 



2. Generalized section from Holyhead Mountain to Menai Straits. 



3. Section across Mynydd Llwydiarth. 



4. Section from Porth Nobla to Aberffraw. 



5. Section in Upper Quarry, Nebo. 



6. Section in Lower Quarry, Nebo. 



In all the sections the signification of the letters is as follows - 



d. Carboniferous. 



c. Cambrian. 



b. Pebidian. 

 ( a 6 . Granitoi elite. 

 I a.. Dark schist. 



a. Gneissic series \ ** f re ? f eiss ' 

 j « 3 . Limestone. 



| a 2 . Quartz schist. 



^ a v Halleflinta. 



/. Faults. 



Discussion. 



Dr. Hicks agTeed with Dr. Callaway that there are two Pre- 

 cambrian series in Anglesey. He differed from the author, how- 

 ever, in regarding the so-called granitoidite as constituting the 

 lowest and not the highest member of the so-called Gneiss 

 series. He stated that some of the breccias associated with the 

 halleflintas contain pebbles of the granitoid rocks, and are therefore 

 of younger age than the latter. He admitted, however, that some 

 of the points must be regarded as in an unsettled state, owing to 

 the faulted condition of the district. 



Prof. Eamsat argued against the principle of identifying rocks as 

 of different ages by their mineral characters as studied by the micro- 

 scope. He maintained that the altered rocks of Anglesey are the 

 metamorphosed representatives of the Cambrian, because the un- 

 altered Cambrian are found striking directly towards the altered 

 strata, and both are overlain by the Arenig. 



Mr. W. W. Smyth also argued against the recognition of a number 

 of different formations on mineral evidence alone without any aid 

 from organic remains. He thought the so-called " gneissose rocks " 

 differ widely from typical gneisses, and that the granitic series 

 belonged to the class of ill-defined granite rocks known in Corn- 

 wall as " bastard granite," the plain Saxon of which was perhaps 

 preferable to the cacophony of " granitoidite." He thought that the 

 evidence brought forward was insufficient to upset the detailed map 

 of the Survey. 



Prof. Bonne r was inclined to agree with Dr. Callaway's interpre- 



