IIETAMOEPHIC EOCKS OF SOUTH DEVOUT. 



719 



South of this valley, the most northerly of the met amorphic rocks 

 are of chlorite-schist, including micaceous bands. This chlorite rock 

 forms the foundation of some of the Bickerton cottages ; it has been 

 worked at one small quarry south of the Bickerton lane, and at an- 

 other on the hillside overlooking Hall Sands, and is of an ordinary, 

 but very well-banded character. 



"We have thus roughly traced the southern boundary of the phyl- 

 lites, and this boundary cuts obliquely across the general strike of 

 the metamorphic beds, so that different members of this series are 

 thrust against the phyUites as we go eastwards. There is, at places, 

 marked variation of dip and strike, and the beds, as I shall describe 

 later, are often disturbed, and dip, in some cases, at a very high angle. 

 Wherever an actual junction is exposed, it emphasizes itself by de- 

 composition of the rocks into a brown iron-stained material. More 

 often the beds thus affected have been completely denuded, and their 

 former place is marked by a small valley or streamlet. 



II. Miceoscope Slides axd Lithological Characters 

 oe the Metamorphic Rocks. 



A. Microscope Slides. 



I examined slides from various parts of the district, and I may 

 perhaps be allowed to add to the full descriptions, given by Professor 

 Bonney, a few notes on specimens which seem to me not quite of 

 the normal type. The metamorphic rocks, as he has stated, may 

 be grouped into two series, essentially characterized by the abundant 

 presence of mica and of chlorite respectively. I found, however, 

 some specimens containing such an amount of both minerals that I 

 have ventured to speak of them as micaceo-chloritic: but if this 

 term is objected to, they can be placed as exceptional forms, partly 

 of chlorite-schist, mainly of mica-schist. These rocks occur at 

 places where there are alternating beds of chlorite- and of mica- 

 schists, and especially along an extensive tract in the north of the 

 area. It is true that chlorite is present in some of the typical mica- 

 schists ; but these differ from the " micaceo-chloritic," even in hand- 

 specimens, and markedly under the microscope ; while in the true 

 chlorite-schists, if the colourless mica is found, it occurs generally 

 in only an occasional flake. 



1. Chlorite-ScJiists. — When we examine, as our first example, a 

 typical slide of chlorite-schist, we see, without magnifying, irregular 

 bands, greenish in colour, alternating with bands of material appa- 

 rently quartzose. By the aid of the microscope the separate consti- 

 tuents can be investigated. The green layers consist mainly of a 

 mass of chlorite aggregated in the modes described by Professor 

 Bonney, generally associated with some brown ferruginous deposit, 

 not identifiable, and with epidote. The epidote may appear in nume- 

 rous small grains, or in larger crystals, some exhibiting cleavage- 

 lines, and showing occasionally a tendency to break up. The 

 chlorite folia are sometimes grouped in a radiate manner; they 

 are generally dichroic, changing from a feeble brownish tint to a 

 deep green colour; by the extinction being parallel to the cleavage- 



