The Devonian Metamorphic Bocks. 31 



that the Beeson Grits merely represent an outcrop, the next series 

 on the south, viz. the Tinsey Head Slates, must be regarded as the 

 highest in the local Devonian sequence. 



This last-named series is interesting from the fact that its southern 

 limit is well-defined by the 'brownrocks' which border the hornblende- 

 epidote-schists (Metamorphic group), and which limit is also more 

 or less easily determinable, west of the Kingsbridge estuary, where 

 the slates are in contact with the mica'schists of the Metamorphic 

 group. It will be remembered that a considerable portion of this 

 series to the east of the estuai'y was included, in the old Survey 

 map, with the Metamorphic rocks. The author now includes with 

 this series the " interlaminated argillaceous, calcareous, and siliceous 

 beds " of Tinsey Head, previously grouped by him with the Beeson 

 beds ; these contain seams of crinoidal limestone, and even Zaphrentis 

 has been found in the Southpool section. He also remarks that 

 the absence of igneous rocks in these beds supplies a further 

 argument against their correlation with those of the Torcross 

 section. 



We may observe that the so-called " Tinsey Head slate series " 

 derives its chief interest from the fact of its being that portion of 

 the Devonian system which is in contact, all along the line, with 

 the Metamorphic rocks. In the days when the doctrine of pro- 

 gressive metamorphism appeared to find favour, the petrography 

 of the various members of this series was the subject of much study. 

 Even now the Director tells us (p. 25) that the mechanical structures 

 of a much puckered rock from Tinsey Head are precisely those seen 

 in the quartzose mica-schists. Doubtless the mica-schist section 

 of the Metamorphic rocks represents a series of arenaceous and 

 argillaceous deposits which may be matched by rocks occurring in 

 the Devonian area. It would be strange if it were not so, when 

 individual fragments of each are selected for comparison. Since, 

 however, Mr. Ussher tells us that the boundary of the Tinsey Head 

 slate series is well-defined, this can only result from an entire and 

 -obvious contrast in the aggregate of the rocks on either side the 

 boundary. Doubtless he has done well to oppose the application 

 of the doctrine of progressive metamorphism in this case, but his 

 resources are not yet exhausted, and he has a second string to 

 his bow, as we shall perceive in the sequel. 



Metamorphic Bocks or Southern Area. 



The rocks of this area, he says, present two well-marked strati- 

 graphical groups— the green schists and the mica-schists. Of these 

 two groups the green schists are believed to be the uppermost. 

 ^'The mica-schists undoubtedly represent a series of sediments of 

 the same general nature as those found in the Devonian area ; and 

 the green schists may unhesitatingly be regarded as an altered 

 series of basic igneous rocks allied to the diabases in composition, 

 and possibly consisting in part of altered tuffs." In assigning 

 a different colour to each of these groups Mr. Ussher has effected 

 a great improvement on the old Survey map, though we are informed 



