362 Dr. R. D. Roberts — On Anglesey Geology. 



hearths of the volcanoes, and afford a simple explanation of the 

 remarkable resemblance which unmistakably exists between plutonic 

 rocks and meteoric stones.^ 



V. — On Some Points in Anglesey Geology. 

 By R. D. Roberts, M.A., D.Sc, P.G.S., Clare College, Cambridge. 



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 gi'anitoidite 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 (GtEol. Mag. 1881, p. 423), as follows: — 



" The altered rocks of Anglesey, the ' metamorphic Cambrian and 

 Silurian ' of the Survey, are shown by a variety of evidence to be 

 older than the Cambrian. Kocks of precisely the same mineral 

 character, showing the same order of succession, and lying on the 

 same strike, are sometimes coloured by the Survey as ' Cambrian,' 

 sometimes as ' Silurian.' True Cambrian and ' Silurian ' strata do 

 indeed occur in many parts of Anglesey, and they have given their 

 name on the map to the metamorphic rocks which happened to he near 

 them (the italics are mine) ; but in no case are there any signs of 

 a transition between the altered and unaltered beds." Turning to 

 the Survey map of Anglesey, it is seen that the northern part of 

 the island is coloured and named " altered Cambrian," the southern 

 boundary of which is a curved line running in a general east and 

 west direction representing a supposed fault, where black shales are 

 in juxtaposition to so-called altered Cambrian. From Dr. Callaway's 

 own words just quoted, especially those italicized, I am bound to 

 conclude that he applies the term metamorphic to this so-called 

 "altered Cambrian" — a part of the " gxiarled series" of Professor 

 Hughes. My criticism upon the above was simple. I wrote (Geol. 

 Mag. 1881, p. 573), that if Dr. Callaway in penning the words "in 

 no case are there any signs of a transition between the altered and 

 unaltered beds," had in view the gnarled series occupying the 

 northern part of Anglesey, I differed from him, because at two spots 

 I had observed a passage between the black shales and the so-called 

 "altered Cambrian." 



Dr. Callaway replies as follows : " Dr. Eoberts discusses the 

 contorted rocks of northern Anglesej^ which he affirms Dr. Callaway 

 has included under the head metamorphic. Again we have, ' This 

 area of so-called metamorphic rock,' and that his readers may be 

 thoroughly impressed with my error, he further on reiterates, 

 ' which Dr. Callaway has included under the head Metamorphic,' and 

 more to the same effect. All this is pure error." I have simply 

 taken the plain and obvious sense of Dr. Callaway's words, and I 



1 Namely, by showing that the principal material of the plutonic and volcanic 

 rocks is of cosmic origin, and that the phenomena of heat, winch occur in these layers, 

 depend on chemical changes to which the cosmic sediment, after being covered by 

 thick terrestrial formations, is subjected. 



