MARKER : LAKE DISTRICT IGNEOUS ROCKS. 
491 
III.— Intrusive Rocks, Younger Suite. 
The granites of the Lake District, with their related dykes, are 
to be referred to the Old Red Sandstone period or to the interval 
following the Silurian, and their intrusion seems to have been con- 
nected in some way with the crust movements which have imparted 
a peculiar character to the district. 
{a) Granites. — There are three considerable masses of granite, and 
these present very different petrographical characters. The 
Skiddaw granite is seen in three distinct areas, which are 
inliers probably of a large concealed mass. In the southern 
area (in Sinen Gill), and the middle and largest one (in the 
Caldew Valley), the rock is a biotite-granite, with only oc- 
casionally a little white mica ; in the northern area (near the 
junction of Grainsgill with the Caldew) both micas are essential 
constituents. The normal rock is a medium-grained granite 
of light grey colour. In Grainsgill, however, it gives place to 
a greisen, composed essentially of quartz and white mica. 
The Eskdale and Wastdale granite is a moderately coarse 
rock, either reddish or grey, with dark mica only. In thin 
slices it is found that microline and microperthite often play 
an important part, and in some varieties of the rock micro- 
pegmatite. The third granite, that of Shap, forms a plug- 
like mass, not, like the other two, an irregular sheet. It is 
a biotite-granite with large red crystals of orthoclase in a 
medium-grained ground. Thin slices show that sphene is 
rather abundant. The rock is never micropegmatitic. A 
characteristic feature of the Shap granite is the frequent 
occurrence in some places of ovoid patches, an inch or two in 
diameter or larger, of a black fine-grained modification, much 
richer in biotite and sphene than the normal rock. 
(b) Quartz-porphyries. — A number of dykes and some sills, especially 
in the area about the Shap granite, are to be referred to this 
suite of intrusions, and are doubtless related to the granite. 
They have biotite as their ferro-magnesian element, and, in 
addition to porphyritic quartz, some of them contain relatively 
large crystals of felspar. 
