Rev. J. F. Blake — Frecambrian Geology. 483 



piece of genuine Archaean." ^ This question Sir A. Geikie answers 

 offhand — by the guidance of petrographical characters — aided by the 

 hiiminocky surface of the ground, which latter, however, is not 

 confined to Ai'cheean gneisses. These petrographical characters I 

 have described,* and state that the rocks have " much the aspect of 

 a Highland gneiss " ; but I do not think these characters may safely 

 " be taken as a guide " to age. There is certainly at this moment a 

 controversy whether they can be or not. 



Sir Archibald then speaks as if no order of succession has been 

 made out in the Anglesey rocks, and as if their metamorphism was 

 the only argument for their Precambrian age in the strictest sense. 

 I would point out that near Beaumaris the Lowest Cambrian has 

 been shown to overlie them. 



But the most important matter is the decision of the age of the 

 rocks of the Northern District which the Director-General now 

 considers to be Silurian. He states in the first place that "the 

 necessity for inserting " the bounding " fault apart from any actual 

 visible trace of its occurrence arose when the conclusion was arrived 

 at that the rocks at the extreme north of Anglesey were essentially 

 altered Cambrian rocks." That is to say, it is a theoretical fault. 

 Dr. Callaway has given evidence for this fault, and my own state- 

 ment is : " In this Northern District a very definite sequence of 

 rocks may be demonstrated, .... We can follow the strike with 

 considerable accuracy. Now, whatever part of the series we may 

 be on from the lowest to the highest, and therefore whatever class 

 of rock is on the northern side of the fault, as soon as we overstep 

 that boundary we are immediately landed in black shales, which 

 have a pretty uniform character throughout. This leaves no alterna- 

 tive but a fault or an unconformity ; if it were an unconformity, the 

 upper group would run parallel to the boundary, which it does not." 

 "Whoever denies this fault, let him answer this argument, and also 

 explain how masses of Cemmaes Limestone come to be found in the 

 basal Silurian conglomerates. 



In fact, however, this line is not in modern language a fault, but 

 a "thrust." The Director adds: "Where the supposed elliptical 

 fault reaches the shore at Carmel's Point, the two groups of rock 

 seem to me to follow each other in unbroken sequence." So they 

 do to me, but things are not always what they seem. At Craig-an 

 Knockan, Eoss-shire, and elsewhere, there is an apparent "conform- 

 able upward succession" between Silurian rocks and the overlying 

 gneisses. The little bands of black slate caught up in the motion 

 in Anglesey are just what we might expect in a similar case. 



Dealing next with the nature of the upper part of the rocks, some 

 of which have yielded fossils, he states that they include " courses of 

 black shale containing Lower Silurian Graptolites"; also "it has been 

 supposed that the higher bands of black shale may also have been 

 brought into their present position by faults" . . . "but this suggestion 

 is completely disproved by the coast sections, which exhibit many 

 thin leaves of black shale, sometimes less than an inch thick.'' 

 1 Q.J.G.S. vol. xliv. p. 497. * Eep. Brit. Assoc, for 1888, p. 32. 



