Keegan: On a Certain Structure in the Lakeland Lavas. 367 
ground-mass exhibits a similar ‘threadwork’ structure, but 
the felspar laths are distinctly discernible with ordinary light and 
a low power, although there is here a decidedly greater and 
more disorderly distribution of the brownish flakes and blotch- 
ings than in the above instance. Again, in a lava (set down in 
the Geological Survey maps as an ash) from Stybarrow Crag the 
same ‘threadwork’ texture is visible in the ground-mass, and 
the tiny laths are clearly visible in the beautiful thin section now 
before me, but the whole is decidedly lighter and brighter owing 
to the comparative paucity of the brown spots, which, however, 
vary much in size, shape, and opacity. Similar remarks apply 
to the more typical augite-andesite of the environs of Derwent- 
Water and, in fact, to all the undoubted Lake District andesites 
which I have had the opportunity of observing microscopically. 
_ On the other hand and contrariwise, a section of a compara- 
tively fresh and unaltered augite-andesite from Himmerich 
Honnef, Rhine, does not exhibit any such ‘threadwork’ structure 
in its glassy ground-mass, nor is there the slightest invasion 
thereof into the felspar crystals, although some of the augite 
Shows traces of it apparently. It would appear, then, from this 
observation that in andesites the augite is the first to yield, i.e., 
the loss of iron and magnesia commences there ; it is otherwise 
in the case of basalts,\where the felspar has been found to 
decompose before the augite and olivine have begun to give 
way. If we examine a thin section of a sandstone, or of an 
arkose, or of an apparently clastic specimen of Hornblende- 
Gneiss from Sutherland, or, most interesting of all, the ‘base’ 
of a genuine volcanic ash, we can detect in none of these 
Structures the slightest symptom of the peculiar decomposition 
phenomena afore-described. The mineral constituents of all 
attrition, seem apparently to be, notwithstanding their venerable 
age and standing, comparatively fresh and unchanged. 
What is here especially broached and advanced in this brief 
Paper is, that the ‘threadwork’ structure herein described and 
due, no doubt, to a process of devitrification, i.e, to the 
formation of definite silicates out of and in the heart of 
a homogeneous glass when an excess of basic matter occurs 
there, may possibly be of service towards supplying some sort 
of clue as respects the real origin of certain rocks, more 
Particularly as to whether they are really ashes or lavas, clastic 
or pyroclastic. 
December 1898. 
