THE 



GEOLOGICAL MAGAZINE 



NEW SERIES. DECADE III. VOL. IX. 



No. VI.— JULY, 1892. 



I. — On Certain Affinities Between the Devonian Eooks of 



South Devon and the Metamorphic Schists. 



(Part II.) 



By A. R. Hunt, M.A. 



{Oontinued from page 247.) 



(PLATE VIII.) 



The Metamorphic Green Eooks and Devonian Volcanics. 



In comparing the quartz-schists with the Devonian sandstones, or 

 grit-bands, several original minerals can be traced from the one to 

 the other, e.g. tourmaline, mica, and at least two quartzes. In the 

 case of the Green Eocks and Devonian Volcanics we are met at the 

 outset with the difficulty, that not a grain or crystal of an original 

 mineral has been recorded with confidence as occurring in the 

 metamorphic Green Eocks ; though there seems to be little doubt 

 that the latter are often metamorphosed diabases. 



The metamorphic Green Eocks seem to be exclusively composed 

 of secondary minerals, of which felspar, hornblende both fibrous 

 and compact, chlorite, and epidote, are the most important. 



The Devonian Volcanics, when hard enough for slicing, are for the 

 most part diabases, in which secondary felspar, the two hornblendes 

 epidote, and chlorite, are here and there developed, to a greater or 

 less extent, by chemical or dynamic action, or by both combined. 

 Microscopic research is thus necessarily limited to the above 

 secondary minerals. 



The relation of the Devonian Volcanics to the sedimentary rocks, 

 the relation of the metamorphic green rocks to the mica-schists, 

 and the relation of the Devonian rocks to the metamorphic rocks, 

 from the stratigraphical point of view, form no part of the present 

 inquiry. Suffice it to say that from the latitude of Dartmouth to 

 Torcross, diabases of Devonian age occur here and there striking 

 roughly east and west ; and that on crossing the metamorphic 

 border-land we find the place of these diabases taken by green rocks 

 of disputed age.^ 



Within the limits of Dartmouth Harbour and Eange we may 

 notice three types of diabase. A porphyritic diabase occurs at 



1 Maps indicating the lines of strike of certain of the Diabases and Green Rocks 

 were exhibited by Mr. W. A. E. Ussher in Section C at the Cardifi Meeting of the 

 Eritish Association, 1891. 



DECADE III. — VOL. IX. — NO. Til. 19 



