AND LEICESTERSHIRE AND THEIR RELATIONS TO GEOLOGICAL STRUCTURE. 133 
basic marginal granite described above. The “ heathen ” consist mainly of a felted 
mass of plagioclase and orthoclase felspars. Quartz is, as a rule, only present as an 
interstitial constituent of minor importance, but occasionally it takes on a poecilitic 
habit as in the basic granite. Dark minerals were abundant and were originally 
represented by hornblende which was quite allotriomorphic, not idiomorphic as in the 
basic granite. The hornblende is now entirely replaced by chlorite in the specimens 
examined. In fact the minerals in the “ heathen ” always present the appearance of 
being more “ weathered” than the corresponding minerals in the matrix. The plagio¬ 
clase is, curiously enough, represented by albite or albite-oligoclase clouded by 
secondary products and possibly itself being of secondary origin. Along the junction 
with the normal granite large orthoclase crystals may extend from the matrix into 
the “ heathen,” and then become packed with small inclusions of plagioclase and of 
ragged chlorite after hornblende. One half of such an orthoclase may belong to the 
coarse matrix and be free from inclusions, while the other half is in the “ heathen ” 
and is full of inclusions. 
Iron-ores are mainly represented by magnetite which has a habit similar to that in 
the normal granite, except that the magnetites in the heathen are smaller and some¬ 
times, but not invariably so, more abundant than those in the granite. The amount 
of magnetite varies between 0'5 and 1’5 per cent., and the susceptibilities are 
sometimes higher, sometimes equal to those of the surrounding matrix. 
The hornblende diorite of Brazil Wood occurs close to the Mount Sorrel granite, but 
its relations to the granite have never been exposed to view.* It is a coarse-grained, 
almost basic, rock (E. 2063) of simple mineralogical composition, consisting almost 
entirely of bytownite felspar and hornblende. The hornblende is of posterior 
formation to the plagioclase, and is partly converted into actinolite. Despite the 
almost gabbroid character of the rock magnetite only occurs in small quantity and, 
as a rule, only in minute grains, though there are occasional larger and more irregular 
masses embedded in the hornblende. The total amount of magnetite is, however, 
much smaller than in the granites. Hence, presumably, the low susceptibility 
(12 X 10“^) of the rock. 
Basic dykes in the Mount Sorrel granite. The granite is cut by a number of^ 
basic dykes. An example from one of the freshest of these rocks seems to have been 
an ophitic dolerite, but it is considerably decomposed, the augite being largely 
replaced by chlorite, calcite, and quartz, while the plagioclase is filled with micaceous 
products along with chlorite, secondary sphene, and epidote. Iron-ores are abundant 
in large crystals giving square or irregular cross-sections, and also as skeletal rod-like 
growths. They appear to be entirely magnetite and constitute rather more than 
3 per cent, of the rock, the susceptibility being 80x10“^ which is approximately 
equal to that of the grey granite. 
W. W. Watts, “ Charnwood Forest,” ‘ Geology in the Field,’ vol. IL, 1910, p. 777. 
