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



8. altered limestone (" calc-flinta"). 



x. epidiorite (altered dolerite sill). 



6. spotted hornfelses, etc. 



5. banded hornfels. 



x. epidiorite. 



4. chiastolite hornfels. 



3. tuff. 



2. altered limestone. 



1. andalusite hornfels (sericitized). 



Intrusive Granite. 

 The beds on the outer edge of the contact zone, below Eartherford, 

 well seen on Ball Hill, are alternations of thin bands of dark, 

 somewhat fissile shale with lighter-coloured massive bands of grit, or 

 rather, perhaps, of hard gritty shale. The softest dark shales 

 sometimes show faint indications of spotting. The same beds, it may- 

 be mentioned, are well exposed in the Simmons Park at Okehampton. 

 The highest beds which are distinctly altered are impure shales, of 

 which an artificial exposure is afforded by an old quarry near the 

 river on the left bank. A study of slices of these rocks does not 

 reveal anything worth noting in the present brief account beyond 

 a mottled appearance due to a cloudy distribution of secondary 

 mica, etc. Gritty beds occur here and there in the river bed 

 intercalated amongst those of a more argillaceous character, and some 

 bands of hard white quartzite are seen. Thin sections of this last 

 show much secondary muscovite. Some of the purer shales are 

 conspicuously spotted, but they do not possess any features of much 

 interest under the microscope. They appear, however, to represent 

 an initial stage in the production of the chiastolite-bearing hornfelses, 

 as the latter show minute chiastolite crystals in the centre of hazy 

 spots, where least crystalline. The chiastolite hornfelses proper, 

 which, unlike the overlying beds, appear to be quite free from gritty 

 bands, are well exposed in an artificial cutting along a track running 

 beside the stream. The beds dip at low angles and are sometimes 

 nearly horizontal. The rock is extremely fresh, and as a consequence 

 the chiastolite crystals are very inconspicuous, so that in the field it 

 looks more like a compact basalt or dolerite than anything else. The 

 altered limestone which underlies it has an outcrop over 100 yards 

 wide. It has a banded structure, like most of the West of 

 England " calc-flintas", and is generally highly silicified and never 

 coarsely crystalline. Nevertheless, it shows occasional seams which 

 contain augite, hornblende, epidote, axinite, and other silicates. 

 Immediately below the limestone comes the upper epidiorite, a narrow 

 band only seen in the actual bed of the river ; it outcrops a little 

 below the junction of the Moor Brook with the East Okement. Then 

 we find somewhat varied types of hornfels, sometimes spotted, and 

 much tourmalinized in places. These extend for a short distance 

 above the Moor Brook junction and have among them, apparently, an 

 altered representative of a band which further east contains 

 cordierite. Just below the Moor Brook there is a very gently inclined 

 thrust-plane among these rocks, which are nearly horizontal for some 



