490 
HARKER : LAKE DISTRICT IGNEOUS ROCKS. 
(b) Microgranites. — A well-known rock of this type forms two 
considerable masses at the foot of St. John's Vale, and is 
quarried at Threlkeld. This is a fine-textured grey rock 
with small porphyritic felspars scattered through it. These 
are oligoclase. Flakes of biotite are also present, and the 
general mass of the rock is a microcrystalline aggregate 
of quartz and felspars. Smaller intrusions of microgranite, 
both dykes and sills, are found in several parts of the dis- 
trict. 
(c) Quartz-porphyries and Quartz-porphyrites. — These are included 
together, because it is scarcely possible to draw^ any line 
here between the truly acid and the sub-acid rocks. Dykes 
and sills are found at numerous localities, and occasionally 
more irregular intrusions (e.g., Wansfell). The rocks 
are in nowise remarkable. Small crystals of quartz and 
felspar are visible in a compact grey or pink ground-mass, 
which under the microscope may be either microcrystalline 
or cryptocrystalline. The ferro-magnesian element, which 
is never abundant, may be biotite or sometimes augite, 
and both are often chloritised. 
(d) Dolerites or Diabases. — The distribution of these rocks is 
sufficiently indicated on the Geological Survey Map. They 
form dykes and sills, and exceptionally a small boss (e.g., 
Castle Head, Keswick). They are mostly ordinary dolerites 
without olivine, often considerably altered with production 
of chlorite, &c. Usually they appear in the field as dull, 
dark-coloured, medium to fine-grained rocks, without 
porphyritic elements. The Castle Head rock, however, 
has porphyritic augites, and is in places micaceous. 
{e) Basic Diorites or Olivine-Diorites. — These rocks are of limited 
distribution, forming irregular, sheet-like or laccolitic masses 
at Little Knott and Great Cockup in the Skiddaw Slate 
area. They have been described as hornblende-picrite, 
and are black, rather coarsely crystalline rocks, composed 
mainly of hornblende. Thin slices reveal spots which 
probably represent destroyed olivine. 
