HARKER : PETROLOGICAL NOTES. 
415 
with among our boulders, though much less common than the 
gnessic and granitoid rocks. Most of them can be matched among 
the minor acid intrusions (chiefly dykes) so frequent in the English 
Lake District, but it would be rash to put them down as coming 
from that region, the petrological characters of these rocks not being 
sufficiently distinctive. We can only say that their nearest possible 
home is among the Silurian rocks of Westmorland. A specimen 
taken about half-way between High Stacks and South Sea Landing 
shows a flinty-looking, dark brown ground, with pink patclies, enclos- 
ing little lath-shaped sections of glassy felspar and clear grains of 
quartz. 
Micro. [1055]. This is the rhyolitic type of quartz-porphyry, 
showing the uniformly cryptocrystalline (" microfelsitic ") ground- 
mass which some petrologists would regard as indicating a devitrified 
glassy rock, though the specimen shows no conclusive evidence of 
an originally vitreous condition. The imbedded quartz-gi-aias are 
rounded, and penetrated by little inlets of the ground-mass, owing 
to corrosion of the crystals by the liquid magma. The felspars, 
both orthoclase and acid plagioclase, are considerably decomposed, 
and secondary patches of greenish material in the rock, crowded 
with granules of magnetite, may represent destroyed augite. 
A small boulder, from the beach at Hornsea, is, to the eye, 
not unlike the preceding, but the ground-mass is more decidedly 
crystalline [944]. Quartz, orthoclase, finely striated plagioclase, 
and apparent relics of augite crystals are present as before, the 
felspars often giving rise to secondary epidote. A few slender 
prisms of apatite are seen in addition. 
No. 508, from the beach at Green Stacks, is a veiy remarkable 
rock. Pinkish-white felspar crystals and smaller crystal-grains of 
quartz are prominent in the .specimen, but their rounded outlines 
and the manner of their occurrence sufficiently prove their frag- 
mental origin, and some of the larger fragments appear to be sub- 
angular pieces of composite rocks. The ground of the specimen, 
though studded with minute crystal-facets, is essentially a dull, 
compact matrix, varying from pale-green to purplish-brown. 
Micro. [1141]. The slide .shows felspars of various kinds to 
