210 



ME. JUKES-BROWNE AND PEOF. HAEEISON 



The complete succession on Mount Hillaby we estimate as fol- 

 lows : — 



feet 



Grey volcanic mudstones 25 



Very fine-grained argillaceous earths, red, pink, and 



yellowish, often mottled 60 



Chalky earths and marls 25 



Calcareo-siliceous earths, with layers of pumiceous 



sand 45 



Pure siliceous radiolarian earths ISO 



Oaleareo-siliceous earths, passing down into chalky 

 earths , 40 



'~> Scotland Beds. 



325 



Checking this by the dip and linear distance, we find the horizon- 

 tal distance between the base and the south-western peak to be about 

 800 feet, and taking the average dip to be 6° this would bring in 84 

 feet, while the difference of level is 190 feet — the total amounting 

 to 274 feet, compared with our estimate of 235 feet. The total 

 thickness would in this way be increased to 359 feet, and may safely 

 be considered as between 320 and 350 feet. 



On the published map only one fault is drawn through Mount 

 Hillaby, for the eastern slope is so obscured by slips and broken 

 debris that its structure is uncertain. The base or limit of the white 

 earths is fairly clear on the road to Mount All, but on the south-eastern 

 slope it seems to run a long way towards Groves. At the time, we 

 took the material seen near Groves to be the result of landslips ; but 

 we are now inclined to think that much of it is in place, and that it 

 lies outside a second fault which curves round to form the boundary- 

 line along the north-eastern side of the hill (see Map, facing p. 202). 

 It is difficult to understand the high dips of the central part of the 

 hiU without such a troughed piece as is shown in the sketch- 

 section, fig. 4, p. 211. 



In such a faulted trough, narrowing southwards, the strata at the 

 narrow end are likely to be pinched and tilted, which will account 



