36 Reviews — Geological Survey — Devonian Meiamorphic Bods. 



unaltered Devonian rocks on the north and the Metamorphic rocks 

 on the south." 



The question then naturally arises : what are the true relations 

 of these two different systems to each other ? It is admitted that 

 Mr. Ussher has not been able to settle the question of the age of 

 the Metamorphic rocks by stratigraphical evidence, yet we are fully 

 prepared to agree with the Director that "his detailed mapping of 

 the green schists and his careful description of the line which 

 separates the altered from the unaltered rocks will be found of 

 value to all future workers in the district." This is perhaps as 

 much as we can expect from an official memoir in explanation of 

 sheets, which are a great improvement on the old Geological Survey 

 map, where an unproved hypothesis was allowed to take the place 

 of a simple record of facts. 



It may be that the theory of metamorphosed Devonians is not 

 altogether forgotten, for we find Mr. Teall observing (p. 45) " that 

 whatever the age of the rocks in the Metamorphic area may be, it 

 is practically certain that they represent a series of arenaceous and 

 ai'gillaceous deposits with which a considerable amount of basia 

 igneous material was associated, and that, so far as original com- 

 position is concerned, they can be matched by rocks occurring in the 

 Devonian area, though the igneous and sedimentary tj'pes do not 

 occur in the same relative proportions." The petrographer thus 

 invites the strati graph ist to perform a feat of legerdemain whereby 

 certain Devonian rocks, such as the Ashprington volcanic series, 

 might by some hypothetical anticlinal arrangement be transported 

 into the requisite position. Since the application of the doctrine of 

 progressive raetamorphism finds no favour with Mr. Ussher, there is 

 nothing left for the advocates of the metamorphism of Devonian 

 rocks but some such explanation as is suggested, rather than 

 put forward, in the diagram on p. 62. Here the distant Middle 

 Devonian, with its volcanics, seems prepared to jump over the 

 whole of the intermediate Lower Devonian and reappear in the 

 extreme south as the schistose series of the Salcombe peninsula^ 

 It is uncertain, however, on which horse the author is prepared to 

 win, for we remember that he has shown a preference (p. 50) for 

 the Torcross group with its attenuated igneous bands. True, this is 

 a much shorter leap, but the horse is only a poor one when he 

 is landed on the other side of the ditch. 



In bidding farewell to this subject we must express a hope that 

 more sober views will ultimately prevail ; and the desperate shifts 

 to which the advocates of metamorphosed Devonians are now 

 reduced seem to indicate that the period is not far distant when 

 mere hypothesis must give way to a more reasonable and probable 

 interpretation of the facts recorded in this memoir. We rejoice to 

 see that even among the local geologists Mr. Harford Lowe 

 (Trans. Devon Assoc, vol. xxiii, p. 523) has taken an essentially 

 reasonable view of the relation of the South Devon schists to the 

 Devonian slates, and some of his suggestions in that connection 

 are well worthy the consideration of professional geologists. Th& 



