The Lurgecombe Mill Lamprophyre and its Inclusions
Quarterly Journal of the Geological Society
RX~rHE~ 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
... w degrees north of east ; it is intrudedE int~ 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, anddisappears 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 outside the aureole of meta-mo~hism 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 pinkish eolour. Weathered surfaces are brown. Towards the margins the rock becomes vesicular, the cavities, sometimes more than an inch long, being filled with a rhombohedral carbonate and quartz, the latter at the centre. In the upper portion of the dyke the cavities are merely in par~ filled with powdel T 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 given 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 baSaI -sections ~hclude acicular crystals (possibly r~ile) arranged in three dh'ections as a sagenite 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 of the q. J. G. S. No. 286. H on May 31, 2016 at University College London http://jgslegacy.lyellcollection.org/ Downloaded from 78 ~R. m o.