H. J. Loice — Rock Differentiation. 345 



A small stream with, south-east course has cut a narrow valley 

 through the softer Culm raaterial contiguous to the igneous rock on 

 the west, and sweeping round the mass to form nearly a right angle 

 continues its course in a north-east direction into the Bovey depression. 

 Thus the igneous mass is cut out of its enveloping sedimentaries on 

 both, the west and south sides, presenting precipitous slopes in those 

 directions. The top and slopes of the rock are grass-covered, but at 

 the shoulder two or three bare masses protrude through the sward, 

 and a small quarry has been worked in it at a lower elevation to get 

 road-metal for the farm tracks. 



The position being near to the edge of the Bovey basin the rocks 

 decline eastward, dipping towards and beneath the Bovey deposits, so 

 that the intrusive, taking the inclination of the invaded strata, becomes 

 immediately buried therein in that direction. In a hollow or combe, 

 however, of the adjoining farm called Stancombe, an adit has been 

 driven in connection with some unsuccessful mineral researches, and 

 among the debris some pieces of a similar igneous rock occur showing 

 that the intrusive had been reached in the excavation and is continuous 

 in that direction. The declivity at the bend of the stream measures 

 some 120 to 140 feet in vertical height, which would also appear to 

 indicate the thickness of the intrusive rock. 



The first specimens of the rock obtained were not found in situ, but 

 from masses used to support a hedge bank along the shoulder of the 

 hill in the usual Devonshire way. Some of these blocks showed a 

 nodular structure, were much weathered, and required considerable 

 labour to obtain a piece sufficiently fresh to be of service for micro- 

 scopical examination. Regarded as another variety of the diabase so 

 common in the neighbourhood, a surprise was in store, when from 

 a number of sections obtained one proved to be entirely different from 

 any of the others. This section from the Lureombe specimen con- 

 tained no augite, but in place of it original brown hornblende. My 

 surprise and interest led me to trouble Dr. Teall in respect to my 

 interpretation of the rock as having the composition of a camptonite, 

 which he kindly confirmed. Several subsequent visits have led to the 

 discovery that the rock is of still further interest by giving examples 

 in micro-sections of not only hornblende as the ferro-magnesian mineral, 

 but of both hornblende and augite in idiomorphic form ; of augite 

 predominating with hornblende as a growth upon it ; and another 

 variation with augite alone having the ophitic habit. These peculiar 

 variations in constitution in the same igneous mass are sufficiently 

 remarkable to receive special notice, if indeed they do not characterize 

 it as a unique instance of diff'erentiation in mass without any evidence 

 of dynamic deformation. 



The sections described in the following detail occur in descending 

 order from the higher to the lower elevation of the rock-mass as they 

 could be obtained in situ, and represent the varying constitution from 

 the upper to the lowest parts of the intrusion as near as can be 

 ascertained. They are classed as I, II, III, and IV, to distinguish 

 the grades of (i) hornblende alone, (ii) hornblende and augite, 

 (iii) augite predominant, and (iv) augite alone. 



Grade I is a coai'sely grained rock, weathering a rusty brown, with 



