152 Dr. R. D. Roberts — On the Geologij of Anglesey. 



II. — Some Points in the Geology of Anglesey. 



ByR. D. Roberts, M.A., D.Sc, F.G.S. ; 

 Clare College, Cambridge. 



IEEGEET that in consequence of the omission of a correction in 

 my letter (Geol. Mag. Dec. 1881), I represented Dr. Callaway 

 as stating that the Nebo rock was granitoidite instead of a part of the 

 granitoidite series. When the letter was written, I was not able to 

 refer to Dr. Callaway's paper for his exact words, and on seeing the 

 paper two or three days later, and finding that I had not given ac- 

 curately the words, I wrote at once to the Editor of the Geological 

 Magazine, asking him to substitute for the word " granitoidite " the 

 "words " Pre-Cambrian rock." By inadvertence this was not done, 

 and as proofs of letters are not usually sent to authors for correc- 

 tion, I found that the alteration had not been made when the 

 December Magazine appeared. Admitting this verbal inaccuracy?, 

 my argument however is not affected. 



Dr. Callaway's words in the paper referred to (Geol. Mag. 1880, 

 p. 118) are as follows : "As confirming the identity of the Anglesey 

 and Twt Hill granitoidite the following fact is of importance. 

 Messrs. Bonney and Houghton have detected at Twt Hill a passage 

 between the granitoidite and a quartzose conglomerate with a south- 

 east dip. I have visited this section, and, having examined the rock 

 inch by inch, I can entirely confirm their identification. There are 

 no signs of a fault between the granitoidite and the conglomerate, 

 and the transition between the two is gradual and unbroken. I 

 have had the good fortune to discover this identical conglomerate in 

 Anglesey. It is exposed in two quarries near Nebo two miles 

 south-west of Amlwch, dipping to the north-west at a high angle. 

 Lithologically it is perfectly indistinguishable from the Twt Hill 

 rock ; the quartz has the same glazed surface, both conglomerates 

 contain disseminated crystals of cubic pyrites and are tinged with 

 the same dingy purple colour. I could find no granitoid rock in 

 these quarries, but the ordinary granitoidite occurs on about the 

 same strike one-third of a mile to the north-east. That this con- 

 glomerate is not Cambrian or Ordovician is proved by the fact that 

 in these quarries black Ordovician (Caradoc or older) shales rest 

 on its upturned edges. It is also associated with bands of quartzose 

 grit as in the Twt Hill locality." 



Dr. Callaway's position is clearly this : 



1. He believes that there is an unbroken transition between the 

 quartz conglomerate and the granitoidite at Twt Hill. In other 

 words that the Twt Hill quartz conglomerate is a part of the grani- 

 toidite series. 



2. He identifies the quartz conglomerate and grit at Nebo with 

 the quartz conglomerate at Twt Hill by their lithological character. 



3. He concludes that the quartz conglomerate at Nebo is Archeean, 

 and a part of the granitoidite series, because black Ordovician shales 

 rest against and on it, as he believes, unconformably. 



I have contended as follows : — 1. That the Twt Hill quartz con- 



