The Devonian Metamovpliic Roclis. 29 



colouration. An attempt has also been made to differentiate the 

 Devonians amongst themselves, and the Director announces that 

 an approximate boundary will be shown, in a future edition, between 

 the Dartmouth Slates and the overlying Meadfoot group. 



In the introduction to the memoir Mr. Ussher gives a table of 

 the rocks in the district, which, as regards the Devonians and 

 Metamorphics, may be abridged as follows : — 



DEVONIAN OR NORTHERN AREA. 



Group. Local Subdivisions, etc. 



Meadfoot axd Lode Beds Tiusey Head Slate Series. 



Beeson Grits. 

 Torcross type. 

 Ringmore type. 



Dartmouth Slates Glossy, green, purple, etc., slates (constituting 



the lowest beds) . 



Some of the above series contain a certain amount of igneous 

 rocks, the basic predominating. 



METAMORPHIC OR SOUTHERN AREA. 



Altered Sedimentary Series... Mica-schists, quartz-schists, interbedded mica- 

 schists, and quartz -schists. 



Altered Basic Rocks Green schists or hornblende - epidote - schists 



(presumably the higher series) . 



Devonian or Northern Area. 



The structure of these rocks is described in the introduction. 

 They are subject to a general east and west strike, though affected 

 by frequent small plications, which tend to obscure the larger 

 curves, and thus local strikes are often at variance with the general 

 strike. In interlaminated hard grits and shales the planes of 

 schistosity are generally bedding planes ; in the argillaceous rocks 

 they are mostly due to cleavage with which bedding accidentally 

 coincides"; gnarling is more especially noticeable in hard grit-shales 

 or interlaminated beds : the effects of gnarling and other structural 

 phenomena are illustrated by cuts in the text. The schistosity dips 

 of the Devonian rocks are mostly at high angles, ranging from 60° 

 to 90°. In three traverses taken from the Metamorphic boundary 

 to the margin of the maps it may be noticed that the dips are either 

 northerly or vertical in the immediate vicinity of the boundary-line, 

 the reverse or southerly dips only occurring at some distance 

 therefrom. 



When we come to the distribution of the Devonian rocks within 

 the area much depends upon the interpretation of an obscure strati- 

 graphy, which has been only too fruitful of divergent opinion, 

 though we may express a hope that the true sequence has at length 

 been revealed to Mr. Ussher. The key of the situation seems to 

 consist in an anticlinal of Dartmouth Slates, the lowest Devonian 

 horizon of this district, which throws off the equivalents of the 

 Meadfoot group on the southern wing of the anticlinal. It is with 

 this southern wing that we are chiefly concerned, since it comprises 

 the principal Devonian series of the district right up to the Meta- 

 morphic boundary. 



