US Bevieics — Geological 8urveij — 



of the southern area, under the symbol k, are marked in the index 

 of colours as " Altered Devonian." This old map bears the Ordnance 

 Survey date of 1809, but wq have not seen a copy wherein the date 

 of the geological information is engraved, neither are the names of 

 the surveyors indicated. This is the more unfortunate because there 

 is a marked difference in the boundary-line as shown on the old 

 and new maps respectively, so that it would have been satisfactory 

 to ascertain who was really responsible for having included such 

 a considerable outcrop of undoubted Devonian rocks within the 

 Metamorphic area in the direction of the east coast. Mr. Ussher 

 duly notices this in his memoir, and suggests that, whilst from the 

 Kiugsbridge estuary to the west coast at Hope, " the boundary is 

 shown with sufficient accuracy to lead one to infer that it was traced 

 on the ground," yet that in tlie eastern district the boundary-line 

 was based on conjecture. This explanation is certainly not very 

 complimentary to the early geological surveyors. 



There is more in this old inaccuracy than, at first sight, meets 

 the eye ; because if the Tinsey Head beds were included in the 

 series of the crystalline schists, such inclusion might seem to favour 

 the notion of progressive metamorphism, which has been a cherished 

 hobby with some of the local geologists, but which is not, so far 

 as the present memoir goes, favoured by Mr. Ussher. His views 

 on this point are particularly recorded in his Report of the 

 recent Excursion of the Geologists' Association (Proc. Geol. Assoc, 

 vol. xvii, p. 125), where he speaks of " the entire absence of any 

 appearance of progressive metamorphism in the Devonian rocks 

 approaching the boundary." It would be satisfactoi'y, therefore, 

 to ascertain who was responsible for colouring the Metamorphic 

 Rocks in the old Survey map as " Altered Devonian," and how far 

 this arrangement was agreeable to De la Beche in the face of his 

 published statements to the contrary. There was, we are well 

 aware, a period in the history ot the Geological Survey when it 

 was the fashion to regard crystalline rocks of unknown age as the 

 altered representatives of formations recognized in the district. This 

 was markedly the case when the Anglesey schists were regarded 

 as altered Cambrian. Since we are unable to assign a date to the 

 appearance of the old Geological Survey maps of South Devon it is 

 not easy to pursue this subject further.^ 



The two sheets of which Mr. Ussher's memoir is explanatory 

 were published in 1898 from resurveys by the author; they luark 

 a considerable improvement on the old Survey maps. Thus the 

 line between the Devonians and the Crystalline Schists is for the 

 most part clearly indicated, and the schists themselves are separated 

 into two distinct series, viz., the mica-schists and the green or 

 hornblende-epidote-schists, each series being shown by a different 



^ We have reason to believe that the Kingsbridge area was sui'veyed by De la Beche. 

 His field maps show two distinct divisions in the metamorphic area, otherwise the 

 geological boundaries are as in the published map. Old Series. It should be observed 

 that there is nothing here to connect De la Beche with the notion of metamorphosed 

 Devonians. The responsibility for this has yet to be disclosed. 



