314 The Rev. Prof. Blake — Base of the Sedimentary Series. 



lower group is much more altered than the upper. He affirms also 

 that the two groups are always brought together by faults. Two of 

 these proofs depend upon the detailed examination of isolated spots 

 or rocks, and after having fully examined them, I have elsewhere 

 given full reasons why they cannot be considered satisfactory. 

 With regard to the third reason, it is generally true that the lower 

 part is more gneissic than the upper, but this alone will scarcely 

 be considered sufficient to establish the independence of two groups ; 

 nor does the author appear to lay great stress on this, but merely 

 uses the terms as distinctive descriptive titles. 



My own researches, which have been pretty exhaustive in every 

 area, with the exception perhaps of the granitic portion in the centre, 

 have shown that the gneissic portion is often slaty, and the slaty 

 portion often gneissic ; that the faults by which they are supposed to 

 be separated cannot- in most cases be traced in the field ; and that there 

 is so intimate a connection throughout between one part and another 

 that they cannot be separated into two systems, though a more or less 

 arbitrary line may be drawn between the upper and lower portion. 

 Taken altogether they form so vast and complicated a system, as first 

 shown by Dr. Callaway, and later by my own more detailed examin- 

 ation (which has led me also to include the rocks of Howth Hill and 

 Bray Head in the same series), as to be comparable in extent and 

 variety of forms, not with one zone of the Cambrian, but with the 

 whole ; and they are equally worthy with it, or with the Silurian, 

 of distinct recognition, notwithstanding that it is at the present 

 moment doubtful how far they are fossiliferous. It is for this 

 reason that I call the whole the Monian system. It is more than 

 probable that this, like every other system, may be usefully divided 

 into several subordinate members, possessing distinct characters ; 

 and it is with such subordinate members that any of the groups 

 elsewhere established amongst Precambrian rocks will have to be 

 correlated, if they really correspond. I scarcely feel sure, however, 

 that any such correspondence has as yet been made out, whatever 

 may be the case in the future. Even so, however, the Monian system 

 is no rival to any other group, being more comprehensive than any. 

 It has been objected to this system that it has neither bottom nor top. 

 As to the bottom, this can only be definitely fixed by finding a lower 

 system still. At present it is the lowest system in the area, and the 

 earth is its bottom ; and if any other is found, it will probably be 

 somewhere in the Highlands of Scotland — but not in Wales. As to 

 the top, in Anglesey itself this is defined by the unconformable 

 Cambrian, but the fact of this unconformity shows that there may 

 be higher parts of it elsewhere. Such a higher part, for instance, I 

 reckon the Bi-ay Head rocks to be, which are separated from Anglesey 

 only by the breadth of the Irish Channel. It has also been objected 

 that if it is distinct from the Pebidian, it becomes equivalent to the 

 crystalline schists, and as such requires no new name. This, how- 

 ever, is to misunderstand its nature ; it is only partly schistose, and 

 the special peculiarity of it is that it unites in one sequence true 

 crystalline schists with ordinary sedimentary rocks. 



