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sees Arvonian — that is all the difference 1 ; whilst Prof. Blake, 

 writing in 1890, stated that, at that time, none of the pre-Cambrian 

 or Monian rocks of Anglesey had been definitely identified with rocks 

 elsewhere. 2 



The Monian controversy now transfers itself to Shropshire. Prof. 

 Blake, it would seem, had suggested that the Longmynd rocks were 

 referable to the Upper Monian. Writing in the spring of 1890, he 

 finds that they are divisible into two groups, of which the lower 

 only can be thus referred. The upper of these two groups, accord- 

 ing to Prof. Blake, represents the true Cambrian, and the junction 

 between the two groups is unconformable. Moreover, the volcanic 

 rocks on the east, associated with Caer Caradoc, are not Middle 

 Monian as formerly supposed by him, but represent the interval 

 between Monian and Cambrian, or, in other words, they are above 

 the lower Longmynd group, and may possibly be on the horizon of 

 the Bangor series, which for him is part of the unfossiliferous 

 Cambrian. It is alleged that this position is supported by detailed 

 stratigraphy. On the other hand, there followed a rejoinder from 

 Dr. Callaway in his paper ' On the Unconformity between the Rock- 

 systems underlying the Cambrian Quartzite in Shropshire.' 



This paper resolves itself into a series of negations as to Blake's 

 position. Thus, the felsites regarded by himself as Archaean have 

 not been shown to be intrusive in the Longmynd rocks ; neither 

 has it been proved that the Longmynd series is divisible into two 

 groups separated by an unconformity. Also, the conglomerates and 

 grits associated with the Uriconian (Caer Caradoc series) are an 

 integral part of that system, and are not of Cambrian age ; neither 

 are the granitic rocks of Shropshire intrusive in the Uriconian. 

 As regards the relations between the Uriconian and Longmyndian, 

 he allowed that it was premature at that date (January 1891) to 

 assign a pre-Cambrian age to the Longmyndian, but he was disposed 

 to favour the idea of a break between it and the Uriconian, which 

 was essentially a volcanic formation, whilst the Longmyndians were 

 characterized by their even sedimentation. He thought that such 

 a change of conditions must indicate a break in time, though the 

 unconformity need not necessarily be very great. He also took 

 the opportunity of pointing out that the occurrence of Malvernian 

 granites and schists in the Uriconian conglomerates indicates the 

 existence of an unconformity between the noncrystalline and the 



1 Geol. Mag. 1893, p. 396. 

 '-' Ibid. 1890, pp. 313, 314. 



