Geology of the South-east of Devonshire. 



471 



greenstone, in others a coarse liornblendic slate, and in both states it alternates with 

 shaly limestone at the base of the great limestone mass of the above-named place. 



Another very instructive instance is to be found in the parish of East Ogwell. 

 The lowest part of the trappean mass is there compact and crystalline, and rests 

 everywhere on slate ; to this follow friable arenaceous beds, occasionally alternating 

 with more solid ones, until they gradually give way to shaly limestone, which is 

 quickly succeeded by the coral reef-limestone, so thickly developed in this and the 

 adjoining parishes. These hornblendic beds swarm with organic remains, chiefly 

 Fenestellee, columnar joints of Crinoidea, and shells of Brachiopoda. 



The contemporaneous trap rocks cannot be separated from the intrusive ones 

 by inspection of mere hand specimens, particularly should they have been taken 

 from the harder beds, for a crystalline arrangement is often as perfect in the one as 

 in the other ; even their included position, among sedimentary rocks, would be 

 accounted for by some geologists (and equally erroneously) as instances analogous 

 to phaenomena which have been described from the coal strata of the North of 

 England. But in addition to the evidence from included animal remains, which 

 is not always available, the contemporaneous trappean beds, from having been ex- 

 posed to precisely the same conditions with the associated deposits, present the 

 same divisional lines both of jointed structure and slaty cleavage, as may be seen 

 in the village of East Ogwell. (PL XLII. fig. 3.) 



Similar contemporaneous trappean beds, the results of subaqueous volcanic action, 

 occur at various intervals throughout the South Devon series : and such eruptions, 

 occasionally accompanied with sheets of molten matter, and the subsequent disper- 

 sion by the action of the waves, as in the recent instance of Hotham Island, suggest 

 that they originated by similar events at that distant period ; submarine banks 

 may thus have been formed where before was deep water, and thus the bed may 

 have become suitable for animal life, and fit spots on which Polyps could fix and 

 raise their structures. 



§ 2. Intrusive. — The intrusive trappean rocks which the slate region of South 



Fig. 13. 



Ground-plan of a dyke intersecting slates near West Down, between Ogwell and Ashburton. 



Devon presents are exceedingly numerous, and their eruption has produced very 



3 p2 



