THE GEOLOGY OF THE ENGLISH CHANNEL. 123 



MICRO-PEGMATITE. 



M. 11a. S. 26° W. Edd., 17-8 miles. 



Red granitoid rock of fine grain, black mica. 



Many felspars clouded entirely with red-brown decomposition pro- 

 ducts. Others, but fewer, almost clear. Some crystals are practically 

 opaque in the centre, clear outside, with successive narrow zones of 

 brown. Graphic structure is well developed ; often where it has 

 invaded a felspar crystal there will be included in it small perfect 

 crystals free from this structure. Eepeated twinning is rather rare. 

 The fluid inclusions in the quartz are very small, most have bubbles, 

 and extremely rarely a cubic crystal occurs. There is a little dark 

 green biotite and some ilmenite. Tlie graphic structure is the great 

 feature of the slide. (Plate VI, figs. 1 and 2.) 



APLITE. 



M. lie. S. 26° W. Edd., 178 miles. 



A fine grained red granular rock with nests of schorl visible in the 

 hand specimen. 



An aplite consisting of quartz and felspar only, except for the tour- 

 maline above mentioned. Structure microgranitic. Felspar red, and 

 somewhat clouded in parts, mainly orthoclase, but plagioclase present. 

 The quartz contains numerous and rather large fluid inclusions, nearly 

 all with bubbles, many with crystal inclusions. Most of the tourmaline 

 is indigo in colour, but some small crystals give brown to blue 

 pleochroism. 



This might be a type rock from Dartmoor. It can be matched 

 in situ in the valley of the Tavy toward and below the lower end of 

 Tavy Cleave, and a precisely similar rock was found as a small boulder 

 resting on the rock bed some hundred feet below the surface of the 

 mud at Keyham Extension Works. 



M. 27x. S. W W. Edd., IS'S miles. 



A schorlaceous aplite very similar to M. lie, but which has not been 

 ndcroscopically examined. 



M. 24g. S. 24° W. Edd., 22*5 miles. 



Granular felsite of rich red colour. 



The section exhibits microgranitic structure. All the felspars are 

 more or less clouded, a few considerably, and in some cases the ordinary 

 optical properties are destroyed. Orthoclase distinctly predominates, 

 but plagioclase twinning is not rare. The felspars show rounded out- 

 lines, marked in some instances by a narrow line of iron oxide, and 

 flakes of hematite occur in some of the crystals. The quartz shows 



