358 Miss I. H. Lowe — Igneous Rocks of Ashprington. 



The petrological characters of all the specimens of diahase 

 collected from the exposures in the area east of Harbertonford, with 

 the exception of that from Austin's Close, are similar. They are 

 fine-grained and porphyritic, showing only such variations as might 

 be found in different parts of the same lava. Thus the rocks show 

 more marked flow structure in some parts than in others, variation 

 in the coarseness of the ophitic structure, and slightly different 

 degrees of decomposition. In the exposures at "New quarry" and 

 Crowdy's quarry the diabase is evidently a lava flow interbedded 

 with ashes, and from the resemblance of the petrological characters 

 of these rocks to those of the old quarry and of the copse south-east 

 of Austin's Close, it can be inferred that the last two exposures 

 are of the same lava-flow. The dip of the beds is gentle and all 

 the outcrops of diabase occur either just above or just below the 

 200 feet level. 



The diabase of Austin's Close differs from the other specimens. 

 It shows no trace of original ophitic structure, and the phenocrysts 

 of felspar are large and well-shaped, although much decomposed and 

 with rounded angles ; they have no definite arrangement. The 

 rock seems to have been more resistant to the pressure which affected 

 the district ; cleavage is not so marked, and the felspars, though 

 often deformed and cracked, are not so distorted and drawn out as 

 those in more cleaved diabases. These facts suggest that this rock 

 was not part of the same lava-flow but originated as a small intrusion 

 shown at N.N.W. in Fig. 4. No junctions with the neighbouring 

 rocks could be found, so that direct evidence was not available. 



The limited extent of the coarse fragmental rock, the size of the 

 included fragments and their silicification, which is doubtless due to 

 the action of vapours connected Avith volcanic action, leads to the 

 conclusion that this rock occupies the position of a small parasitic 

 vent (see Fig. 3). Similar vents were probably numerous in the 

 Ashprington area, and if so, this would account for the extensive 

 development of the volcanic rocks in this district. 



We have, therefore, near Harbertonford, evidence of a lava-flow, 

 a great preponderance of ash, a small vent and a small intrusive 

 mass. This is probably typical of the occurrence and origin of the 

 spilitic volcanic rocks in the larger Ashprington area, whose develop- 

 ment was initiated early in the Devonian period in a district that 

 was undergoing gentle subsidence. Thus these rocks give an 

 additional illustration of the view expressed by H. Dewey and 

 Dr. Flett, 1 and more recently by Dr. A. Harker, 2 as to the connection 

 between the petrological characters of the spilites and the conditions 

 under which they were formed. 



1 Geol. Mag., 1911, p. 246. 



2 Quart. Journ. Geol. Soc, vol. lxxiii, p. lxixviii, 1917. 



