420 F. P. Mennell — The Northern Margin of Dartmoor. 



chiefly hornblende and sometimes a fine felt of wollastonite fibres. 

 One variety shows in thin slices beautiful rosette-shaped sections of 

 pink garnet scattered through an aggregate of pale-green hornblende 

 crystals. Fragments of chiastolite-bearing rocks are to be found 

 below the quarry and also occur plentifully on the fields to the east. 

 These no doubt link up with the uppermost beds containing that 

 mineral in the Okement section. Above the camp masses of tuff are 

 the most conspicuous feature, and obscure the other beds until the 

 andalusite hornfels is reached. 



Better exposures are to be seen along the Redaven brook, which 

 runs from the foot of Yes Tor to join the West Okement above the 

 village of Meldon. The succession of andalusite hornfels overlaid by 

 limestone and succeeded in turn by tuff, which is again covered by 

 blackish and banded types of hornfels, is as clear as in the Taw 

 Valley along the banks of the Redaven, and also those of the "West 

 Okement. It may be traced beyond the village of Lake, where the 

 beds begin to pass out of the contact zone, and lower horizons are 

 represented along the edge of the granite. These latter are of a 

 more gritty character, generally speaking, though they include what 

 may be altered radiolarian chert above Lake Viaduct, and crystalline 

 limestone on the north-western slopes of Great Nodden, where we 

 encounter the rocks described in my paper on the Lyd Valley. There is 

 probably more than one fault in this neighbourhood, of which I was 

 not able to complete the examination. 



Returning to the Meldon area, the well-known intrusion of fine- 

 grained topaz-bearing granite or granophyre may be noted as intrusive 

 in what is here called the banded series, which is unusually calcareous 

 at this point. Further west, in the cliffs overlooking the Okement, 

 veins connected with it invade the lower tuff. Above these rocks 

 comes the upper limestone, which has been worked for lime in a large 

 quarry now abandoned, but which thins out to the east into narrow 

 seams interstratified with shale. Westward it is represented along 

 the strike by coarsely crystalline calc-flinta, full of interesting 

 minerals, which affords a good instance of undoubted introduction of 

 silica in the course of the contact metamorphism of limestone. These 

 beds are in all probability the same as worked in the large quarry 

 near Bridestowe, outside the metamorphic area. It may be noted 

 that wollastonite, rare further east, is occasionally abundant in the 

 upper limestone, while the lower limestone is sometimes converted 

 into idocrase-wollastonite rock, which is also to be found among the 

 numerous xenoliths in the " aplite " quarry. Datolite and scapolite 

 are also known to occur. In this section chiastolite-bearing rocks 

 seem again to be the uppermost sediments that have undergone 

 marked alteration. The upper tuff, first described by MacMahon, 

 outcrops near the railway and is well exposed on one side of the 

 viaduct. No epidiorites are seen along the lledaven, but a narrow 

 sill, much tourmalinized, is seen in the big quarry worked by the 

 railway company for ballast a little to the east, as to the horizon 

 of which I am by no means certain. 1 A much more prominent basic 



1 Restrictions connected with the War prevented me re-examining the quarry 

 or inspecting the railway cuttings. 



