146 HARKER: ANCIENT LAVAS OF THE LAKE DISTRICT. 
small porphyritic crystals. All the upper sheets are of this type, 
and these show a very marked ‘flow-structure.’ The hypersthene 
is indicated only by minute black spots. In thin sections, however, 
it is easily recognised, though quite altered into bastite. With it 
occur some augite, magnetite, etc. These compact basalts have 
normally a dark colour, but sometimes, as in Charlton Gill, they 
should be noticed that a vesicular or amygdaloidal character is found 
in some of the flows, or, rather, near their upper and lower surfaces. 
By far the greater number of the Lake District lavas belong to 
the intermediate division, giving 59 to 61 per cent. of silica on 
analysis, and having a specific gravity 2°65 to 2°7. ey are 
pyroxene-andesites, the pyroxene being sometimes a rhombic one 
(hypersthene), sometimes a monoclinic (augite), or the two often 
occurring together. These minerals are not in large enough crystals 
to be easily detected by eye, though the dark patches due to their 
decomposition-products may often be observed. Porphyritic felspars 
are usually present, sometimes rather crowded and up to a quarter of 
an inch long, but more usually scattered, and often minute. The 
ground-mass has a very compact look, with usually either a pale 
greenish or a dark grey tint. Many of the flows are vesicular, or 
have their uppermost and lowest layers vesicular; and the vesicles 
may reach considerable ee as, for instance, on some parts 
of Grange Fells. They are commonly filled with chalcedony, agate, 
calcite, see sibetiaibess tn etc., in concentric layers. 
the andesites have a special character in the occurrence 
of tittle id garnets. The occasional presence of garnets in the 
Lake District rocks is very striking. They occur in basic, inter- 
mediate, and acid lavas alike, and in the ashes and _ breccias 
of the less resisting rocks in the Lake District have been 
more or he affected by profound mechanical disturbances, which 
operated after the Silurian period, and they have thus received 
a cleavage-structure. In some places the stresses produced have 
been very intense, and even hard lavas have become crushed and 
eaved. This is very evident in the case of amygdaloidal lavas like 
the andesites near the Gates of Borrowdale, the vesicles being in 
places flattened into mere films; but compact non-vesicular lavas 
have also become cleaved, and some have even been worked as slates. 
The acid /avas occur at more than one horizon among the 
andesites, but they are most developed at the summit of the whole 
Naturalist, 
