Vol. 60.] ROCKS or THE BORROWDA1E VOLCANIC SERIES. 103 



of chlorite-flakes, a minute granular aggregate, and comparatively - 

 large pieces of a brownish isotropic mineral with occasional tri- 

 angular outline. This has rather the appearance of sphene, but 

 the mineral is isotropic: it maybe one of the spinels — picotite, 

 perhaps. In the next stage we find a highly-refracting mineral with 

 peculiar blue and bluish-green polarization-tints, massed round the 

 grains of spinel. This mineral has a higher refractive index than 

 the spinel, and seems to be a garnet of non-isotropic character. 

 The brownish spinel loses its colour, and changes into an opaque 

 iron-ore. This becomes gradually absorbed, and a true isotropic 

 garnet is produced. 



The non-isotropic garnet might very well be grossularia, 

 for the polarization-tints are characteristic of that mineral, and 

 there is no difficulty in this assumption when the large amount 

 of plagioclase present is considered. It is a curious fact that the 

 larger masses of garnet show no trace of the spinellid mineral, but 

 at their outer edges pass into the non-isotropic garnet. That some 

 such change as this does occur seems fairly certain, but it is very- 

 desirable to obtain confirmation by chemical methods. 



The flakes of biotite in this rock show numerous pleochroic halos, 

 and apatite seems to occur as a product of contact-metamorphism. 



The production of garnets in the ash is limited to fragments 

 caught up and enclosed by the intrusive rock. The purple ash does 

 not usually contain them. It appears therefore somewhat absurd 

 to ascribe the perfect garnets of Illgill Head and Sty-Head Tarn 

 to the same metamorphic action. 



Lavas, possibly of the Eycoti type, come close to the garnets just 

 north of Stony Tarn, and also at Brockshaw (rill. The product is a 

 purplish rock, with greenish-white felspar-substance. Under the 

 microscope aggregates of mica- flakes are seen to be very common, set 

 in a mass of plagioclase. Xearer the junction north of Brockshaw 

 Gill the purple colour is lost, and we get a grey rock with occasional 

 garnets (3930). This rock is not found further than 2 feet above 

 the line of junction, and passes into the purplish rock above. 



A metamorphosed Eycott Lava in Oliver Gill shows the production 

 of brown mica and a light greenish hornblende. 



A purplish ' streaky '-like rock south-east of the head of Wast- 

 water exhibits minute biotite-flakes, aggregated round ragged patches 

 of iron -ore (3793). Biotite is often observed in association with 

 a greenish hornblende. Aggregates of quartz-grains are frequently 

 associated with biotite in these rocks, but there is nothing to show 

 that such aggregates are the result of metamorphism. 



Coming to the examples of contact-metamorphism by a small 

 intrusive mass, we find these in the ash above the intrusive 

 garnet-bearing rock of the path to Dock Tarn, and in a similar ash 

 at the upper limit of the intrusive sill on liosthwaite Fell. The 

 phenomena are the same in both, consisting of a production of 

 black crystals and flakes of chlorite. In the lvosthwaite-Fell 

 metamorphosed rock (3778) irregular flakes of greenish chlorite are 



