Search

A few words of explanation are required to enable those not familiar with the Carnarvon district fully to appreciate the importance of Prof. Bonney's confirmation of the view respecting the age of the Twt Hill conglomerate first put forward by Prof. Hughes, and afterwards advocated by myself. The controversy possesses a general interest, apart from the fact that the stratigraphical position of a certain bed is now fixed, inasmuch as it shows how cautious it is needful to be in dealing with these ancient rocks without the most thorough and careful mapping of the whole area.

A discussion has more than once arisen, in the course of the last two years, respecting the true position or the quartz conglomerate exposed near Twt Hill, Carnarvon, which was first described by Prof. Bonney and Mr. Houghton in the Quarterly Journal of the Geological Society, vol. xxxv. p. 321. The typical quarry is situated on the S.E. side of the ridge, close underneath Twt Hill, and the exposure there shows the quartz conglomerate in juxtaposition to the granitoid rock that constitutes the axis of the ridge. The authors describe a passage between the granitoidite below and the conglomerate above, and state that the latter ?passes lip into a rock which has some resemblance to the bottom rock? (granitoidite). In the GEOL. MAG. for March, 1880, p. 118, Dr. Callaway writes: ?Messrs. Bonney and Houghton have detected at Twt Hill a passage between the granitoidite and a quartzose conglomerate with a S.E. dip. I have visited this section, and having examined the rock inch by inch, I can entirely confirm their identification.?

I Regret 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 granitoiditeinstead 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 accurately 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 correction, I found that the alteration had not been made when the December Magazineappeared.

The following facts were observed during a visit to Anglesea and Carnarvonshire in company with Prof. Hughes in the early part of July. They still further bear out and confirm the evidence given in a short communication to the Geological Magazine for May, 1881, bearing upon the position of the Twt Hill conglomerate. Among the arguments brought forward against the Cambrian age of that conglomerate, much stress has been laid upon the absence fromit of felsite pebbles, which are so characteristic of the Cambrian conglomerate in the neighbourhood of Bangor.

In my former short communication on this subject, I referred only to the question of the age of the quartz conglomerate at Nebo, which Dr. Callaway mistook for a part of the granitoidite series, but which Prof. Hughes has shown to be Cambrian conglomerate. What Dr. Callaway says in reply may be seen at page 287. The second point which I stated I should deal with later, I now take up. Dr. Callaway wrote (GEOL. MAG. 1881, p. 423), as follows :?