385 



tion angles are high and indicative of labradorite or anorthite. 

 M. BARROIS has recorded the occurrence of felspar in a sandstone of 

 Brittany. In this case the felspar is due to an impregnation of the 

 sedimentary rock with granitic material. It is found only in the 

 immediate neighbourhood of the granite. 



Iron ores. The amorphous iron oxides and hematite of the un- 

 altered sediments are converted into magnetite by igneous intrusions. 

 The magnetite may occur in grains or octahedra. 



Quartz. This mineral undergoes recrystallization in the process ol 

 contact metamorphism. In the arenaceous rocks and the clay-slates 

 the original ^boundaries of the quartz grains disappear, so that the 

 mineral loses all traces of its clastic origin. The brown mica, so 

 characteristic of contact metamorphism, sometimes occurs as inclusions 

 in the recrystallized quartz. 



Tourmaline. This mineral is very characteristic of contact, meta- 

 morphism in certain districts. It may occur as idiomorphic crystals, 

 as large irregular crystalline masses moulded on the other constituents, or 

 as small grains. It may be brown, blue, green or nearly colourless. The 

 larger crystals sometimes show a very perfect zonal structure. Minute 

 inclusions in the tourmaline are often surrounded by borders of a 

 colour different from that of the main mass. Thus in brown 

 tourmalines the borders in question are often green. 



DESCRIPTION OF THE ROCKS. 



West of England. The granite-masses of Devon and Cornwall have, 

 in certain places, produced important effects on the sedimentary rocks 

 of the district. These effects are by no means uniform in their character. 

 Sometimes they may be traced to considerable distances from the granite, 

 at other times they are very local and unimportant. It is an unfortunate 

 circumstance that the distribution of the different varieties of contact- 

 rocks is not delineated on the Survey Maps. Each granite-area is sur- 

 rounded by a wash of colour of uniform width, which is supposed to 

 represent the zone affected by the granite. As a matter of fact the 

 metamorphic zone is not of uniform width, and therefore the colour which 

 is supposed to indicate it is somewhat misleading. That the rocks in the 

 immediate neighbourhood of the granite are, in many cases, different from 

 the normal rocks of the district was noticed by the earlier writers on the 

 geology of the West of England. Thus Dr. BOASE W proposed the terms 

 cornubianite and proteolite for certain varieties of rock occurring near the 

 junctions of the sedimentary rocks and the granite. The definitions 

 which he gives of these terms do not, however, enable us to use them 

 with any degree of precision at the present day. The term cornubianite 

 has been used to a certain extent by continental writers, but not in any 

 very consistent sense. The term proteolite has never made any way in 

 petrographical literature. Dr. BONNEY has proposed < 2) that the term 



(1) On the Geology of Cornwall. Trans. Roy. Geol. Soc. Corn., Vol. IV., p. 394. 



(2) Q.J.G.S., Vol. XLII. (1886), Proc., p. 104. 



