part 2] THE METAAIORPHOSED ROCKS OF THE START AREA. 189 



.nature of the Start group of rocks, they are not without interest 

 from a petrographical point of view, more especially in regard 

 to the mineralogical changes associated with progressive meta- 

 morphism. 



These rocks occur characteristically at the upper and lower 

 horizons of the Green-Schist hand. They are particularly well 

 developed in association with the northern band, as at Ilbertstow 

 Point and Scoble Point. Within the Green-Schist band they are 

 abundant at Langerstone Point, and near Limebury Point and 

 Leek Cove, as also at other localities. 



As a group they contain the following minerals : — chlorite, 

 epidote, albite, calcite. sphene, quartz, muscovite, rutile, iron-ores, 

 garnet, and hornblende. Apart from the abundance of muscovite 

 and quartz in them, and the presence of rutile and garnet, these 

 rocks are differentiated from the Green Schists by their very 

 variable composition. This variability of composition is not 

 infrequently shown within single sections cut for examination 

 under the microscope. 



Petrographically, two types can be distinguished : — 



(a) Qiiartz-muscovite-chlorite-albite-schists. 



(6) Quartz-muscovite-chlorite-albite-garnet-schists. 



(a) The propei'ties of the first type can be gleaned from a 

 description of a rock occurring as a band among the Green Schists 

 .at Limebury Point. Macroscopically, it has the habit and 

 appearance of the normal mica-schists, glistening folia of muscovite 

 and chlorite being separated by quartzose layers. 



Under the microscope, the muscovite and chlorite are seen to be 

 intimately intergrown, the latter appearing in pale-green pleo- 

 chi'oic tints with low interference-colours, an abnormal brown tint 

 being not uncommon. These layers of muscovite and chlorite 

 have a highly sinuous development. Between them lie quartz and 

 albite in interlocking grains, the quartz frequently showing 

 undulose extinction. 



The accessory minerals are titanite, epidote, and iron-ore (now 

 haematite and limonite). Yellow prisms of rutile are occasionally 

 abundant. 



(J/) In the next type, garnet appears. The best development of 

 these rocks occurs among the Green Schists in the Prawle mass at 

 Langerstone Point. The garnet is present in well-shaped dodeca- 

 hedra, reaching 1/16 inch in diameter. In other respects these 

 rocks do not differ materially from the type already mentioned. 

 They show considerable variation in their quartz content. Sphene 

 and epidote are often abundantly developed in association with the 

 muscovite-chlorite intergrowths. Apatite is a common accessory. 

 The idioblasts of garnet reach 1'5 mm. in diameter, showing 

 quadrate or hexagonal outlines, and are pinkish in colour. They 

 •are also isotropic. Poikiloblastically epidote or clinozoisite, and 

 less commonly sphene and quartz are found within its borders. 



