ES ncn Pe, 
Cay 
366 Keegan: On a Certain Structure tn the Lakeland Lavas. : 
decided dogmatic opinion anent their structure and origin. 
Even when we hammer off a chip of what to all appearances is 
a fresh and ‘clean’ bit of rock, we are all the more disappointed 
when a micro-section thereof is prepared, and we try very hard 
to make out exactly what is to be seen therein under the 
microscope. Judging from my own experience, I should say 
that the majority of investigators of these old structures are at 
first very much disposed to mistake lavas for ashes, and not 
vice-versA. The fault may be set down first to not having the 
sections thin enough, and secondly to want of experience. In 
many cases I believe it is next to impossible to decide definitely 
whether a given rock is an ash or belongs to a flow, unless the 
section is not only as thin as possible, but is also as uniformly 
thin as can be made. The ‘hyalopilitic’ structure, i.e., the 
felted aggregation of slender laths and microlites of felspar in 
the ground-mass of the andesitic lavas, is sometimes so com- 
pletely obscured and blotted out, so to speak, by what we may 
call the parallel flaky or confused felted aggregates, which seem 
to attend the metamorphosis of every part of the rock, that it 
can be discerned only with difficulty by the help of high 
magnification and crossed nicols. And yet, unless we are clearly — 
satisfied that such structure exists, or has existed, we never can 
be sure whether we have before us a lava or an ash. For — 
thin and uniform section, the ged a Sai crystals of felspar are 
tolerably well preserved, although ‘streaky’ and containing 
irregular patches of a strong, doubly-refracting muscovite, and 
most of the augite crystals are still sharp in outline being only 
partially changed externally; nevertheless, the ground-mass 
viewed with ordinary light seems to be a completely uniform 
fine threadwork in which are imbedded innumerable minute _ 
granules at regular intervals—the whole very comparable to the _ 
aspect of a vegetable nucleus prepared and stained and observed — 
with very high magnification; all over this threadwork, which 
seems to invade the felspar and augite crystals as well, there is 
a very even distribution of not very similar sized patches or oo 
blotches of a light to dark brown matter which seems to be ° 
_ partly chloritic and partly ferruginous. When, however, we 
use a eas igher objective and have crossed nicols, we then 
observe, though not very definitely, small tiny laths and 
- microlites with their long axes lying mostly parallel to thelonget 
_ sides of the porphyritic felspars. In another lava collected from — 
the same hill screen, but about one mile further south, the 
Naturalist, 
