﻿part 2] 



THE LTTRGECOMBE MILE LAMPROPHYRE. 



77 



6. The Lurgecombe Mill Lamprophybe and its Inclusions. 

 By Herbert Gladstone Smith, B.Sc, F.G-.S., Demon- 

 strator in Geology in the Imperial College of Science and 

 Technology. (Eead May 10th, 1916.) 



[Plates VIII & IX.] 



Bather less than a mile north of Ashburton (South Devon), on 

 the eastern margin of the alluvium of the Yeo, which here flows 

 past Lurgecombe Mill, an excavation has recently been made in the 

 hope of obtaining a workable quantity of road-metal. The rock 

 which formed the object of the working is a biotite-lamprophyre, the 

 existence of which does not seem to have been previously recorded. 

 The dyke is about 9 feet wide, and is exposed for a length of 

 144 feet in a direction a few degrees north of east ; it is intruded 

 into Carboniferous shales (the thrust, as mapped by the officers of 

 the Geological Survey, runs east and west about 100 yards south 

 of the quarry) with which are interbedded occasional cherts. 

 An examination of the country in the immediate vicinity does not 

 reveal any prolongation of the intrusion ; it plunges underneath 

 the shales of the rising ground at its eastern end, and disappears 

 under the alluvium on the west. It is not exposed in the bed of 

 the stream, although here the alluvium has been swept away, but 

 it may take a sharp turn and pass under the stream above the 

 weir. The shales in contact with the dyke are merely indurated ; 

 no new minerals have been developed in them in consequence of 

 this intrusion, and the locality is otxtside the aureole of meta- 

 morphism of the Dartmoor granite ; the nearest outcrop of this 

 granite is nearly 2 miles away. 



The intrusive rock is, for the greater part, compact and fine-grained 

 in texture and dark grey in colour. Abundant small flakes of 

 biotite can be recognized, and there are many small patches of 

 a pinkish colour. Weathered surfaces are brown. Towards the 

 margins the rock becomes vesicular, the cavities, sometimes more 

 than an inch long, being filled with a rhomboheclral carbonate and 

 quartz, the latter at the centre. In the upper portion of the 

 dyke the cavities are merely in part filled with powdery limonite. 

 Crystals of pyrite are to be seen near the margins of the dyke, and 

 here the alteration has resulted in the development of .a dominant 

 green colour, though the biotite is still recognizable. 



In thin sections the biotite is conspicuous. It transmits various 

 shades of brown, the deepest colour, being exhibited by the^ basal 

 sections. Many of these are idiomorphic, and are darker brown on 

 the edges ; other sections are frayed, and show a cleavage parallel to. 

 the length. The mineral is remarkably fresh, but some of the few\ 

 altered basal sections include acicular crystals (possibly rutile) ' 

 arranged in three directions £LS Hi Set genite web ; these directions are 

 at right angles to the edges of the hexagon — the directions of the 

 rays of the pressure-figure of micas. The pleochroism is 



Q. J. G. S. No. 286. ^S» on,an 



' fa -DEC 121917 \£ 



