Correspondence — Prof. T. G. Bonney. 479 



Like all instruments of research, this one of shape must be used 

 with common sense and the surrounding circumstances taken into 

 consideration, but when as on Moel Tryfaen extremely rounded and 

 polished grains of quartz are found amongst a great mass of very 

 angular material they may be treated as erratics. No rock in the 

 neighbourhood could yield them, and to the educated eye they at 

 once proclaim their sea-origin, whatever mode of transit may be 

 theoretically provided for them according to the proclivities of the 

 geologist. 



I am glad of the opportunity of reiterating these views first 

 brought forward in a paper recently read before the Geological 

 Society. T. Mellard Keaue. 



Park Cokner, Bluxdellsands, Sept. 1th, 1892. 



THE EOOKS OF SOUTH DEVON. 



Sir, — Now that Mr, A. R. Hunt's three-months -long dissertation 

 on the Devonian Rocks of South Devon has come to an end, I may 

 ask space for a very few words, as I do not intend to discuss the 

 subject in detail. 



He attaches importance to mineral coincidences between the 

 schists and the admitted Devonian rocks. Some of these, such as the 

 iron-ores, seem to me very much of a Monmouth-Macedon type ; 

 others to be more naturally explained by supposing that the latter 

 have derived some of their materials from the former or a kindred 

 crystalline group, an alternative which seems to me inadequately 

 discussed in his paper. 



As I have always held that the dark mica-schists were once 

 sediments, as the Devonian phyllites have been, and I have never 

 denied the possibility that some of the green chlorite -schists 

 originally might have been basic igneous rocks, parts of Mr. Hunt's 

 arguments do not afi"ect my position. 



From Mr. Hunt's paper I infer that he is not aware that a schist, 

 g,fter crushing (particularly if dark in colour), is sometimes very 

 difficult to distinguish from a much-squeezed dark slate ; also that 

 some other crushed crystalline rocks simulate squeezed grits. The 

 difficulties are local, and generally can be overcome when you know 

 what to look for, but they are so real that I always hesitate to 

 express an opinion on microscopic slides when I have not seen the 

 rock in the field, and even then, once or twice, when the outcrops 

 were scanty, have been unable to come to a conclusion. 



I have never denied that what it is now the fashion to call 

 dynamometaraorphism has greatly modified both the schists and tl e 

 Devonian rocks, but, in calling attention to it, I pointed out that the 

 one set " went into the mill " as schists, the other as clays. I do 

 not find that Mr. Hunt has adequately discussed this very important 

 matter. 



During the nine years which have elapsed since my paper was 

 written, I have many times examined both my own and other 

 specimens from South Devon, and have had unusual opportunities of 

 studying, in other regions, similar rocks and some sections which 



