56 Dr. C. Callaway — Geology of Anglesey. 



two points (at any rate) .... a distinct passage can be seen anrl 

 traced inch by inch from the fossiliferous shales to the beds marked 

 ' altered Cambrian ' on the Survey Map." At one of these localities, 

 " the two series can be seen passing into one another, the one 

 dipping under the other." Dr. Roberts contents himself with a 

 "passage" at "two points." I could have told him of dozens of 

 such " passages." Every one familiar with Archeean work knows 

 that talcen alone they prove nothing. The unaltered beds are let 

 down by faults, and, when the dips are the same (which is often the 

 case in the north of the island, though less frequent in the west), 

 the newer rocks appear to pass conformably under the older. It is 

 unsafe to affirm a passage between two groups, unless there is a 

 gradation in mineral characters and, when one is altered, in state of 

 alteration ; but Dr, Eoberts asserts neither of these conditions. It 

 needs double caution in working on his method when the upper 

 group is more highly altered than the lower, since the theorist must 

 be prepared to consider why the metaraorphism did not affect the 

 lower series, and why, in its supposed downward progress, it stopped 

 abruptly. During several months of close work, undertaken at 

 intervals extending through the last three years, I have examined 

 scores, if not hundreds, of junctions between altered and unaltered 

 groups in Anglesey, and the line of separation was invariably clear 

 and sharp. It is incredible that such should have been my ex- 

 perience, had a true passage existed. 



It is not necessary to review the general question of the Arch^an 

 age of the altered and contorted rocks of Anglesey. I will content 

 myself with stating that when Dr. Roberts has thorougJily explored 

 the island, he will find that the Cambrians contain fragments 

 derived from his "gnarled series." This will place him in a rather 

 awkward dilemma. 



My critic's confused views of the Anglesey Archeeans appear to 

 have originated in the assumption that all his " gnarled " rocks 

 belong to one series. I have given reasons for believing that there 

 are tivo contorted groups ; the one newer than the granitoidite, and 

 hypometamorphic ; the other underlying the granitoidite, and meta- 

 morphic. The latter rocks occupy the area west of the Menai 

 Straits, a part of the Central Zone, and the west of the island as far 

 north as the Forth -y-defaid fault. They differ widely from the 

 northern group and the series west of Malldraeth Marsh, in both 

 mineral characters and degree of alteration. The rocks of the north 

 are by my critic's admission not entitled to be called " metaraorpliic," 

 and that I am not alone in applying that term to the Menai and 

 Holyhead schists will appear from the following quotation from a 

 letter by Prof. Bonney, F.R.S., whose competence as a microlitholo- 

 gist will hardly be questioned. 



" I have not yet seen enough of the extremely difficult geology of 

 Anglesey to enable me to commit myself to any opinion beyond the 

 assertion that this island contains a large tract of metamorphic rock 

 of Pre-Cambrian age; neither will I venture to discriminate between 

 the vai'ious rocks which may have been included in the 'gnarled 



