CHAP. XLVIII 
THE ACID SILLS 
433 
durable, stand out above the decomposable surface of the containing rock, 
and show their quartz-blebs and felspar crystals on the weathered surface. 
Perhaps the most striking feature of the granophyre sills of Skye is 
their general association with thinner basic intrusive sheets between which 
they have insinuated themselves. This characteristic structure, pointed 
out by me in 1888, has recently been more minutely mapped in the progress 
ol the Geological Survey. Mr. Harker has found the typical arrangement to 
be the occurrence of a thick sill of granophyre interposed between two sills 
of basalt, each of which is usually not more than six or eight feet thick. 
M here the granophyre has been intruded independently among the Lias 
Fig. 372.— Section across the Granophyre Sills at Loch a’ Mhullaicli, above Skulamus Skye 
formations, it does not assume the regularity and persistence which mark it 
where it has followed the course of basic sills. 
1 he acid rock, Mr. Harker observes, “ is invariably the later in- 
ti usion, tor it sends narrow veins into the basalts, metamorphosing them 
to some extent and frequently enclosing fragments of them. These frag- 
ments are always rounded by corrosion, and show various stages of dis- 
solution down to mere darker patches as seen by the naked eye. Such 
inclusions and patches are found in the marginal part of a granophyre, 
where no continuous basalt occurs, but where the acid magma has evidently 
in places completely destroyed the earlier basic sheets between which it was 
forced. It seems probable that in all cases a certain amount of solution of 
the basalt by the granophyre magma took place at their contact, facilitating 
the injection of the later intrusion and accounting for its persistent choice 
of the contact-plane of two basalt-sills as the surface offering least resistance 
to its injection.” 
These observations throw fresh light on the remarkable original regularity 
and persistence of the basic sills. Where one of these sills disappears 
abo\e or below a granophyre sheet its probable former presence is often 
indicated by corroded fragments of the basic in the acid rock. Mr. Harker 
remarks that the acid magma seems to have been “ in itself less adapted 
than the basic to follow accurately a definite horizon and to maintain a 
uniform thickness in its intruded sheets, but could do both when guided by. 
a pre-existing basalt-sill, or especially when insinuated between contigu- 
ous basalt-sills.” The corrosive action of the acid magma on the surface^ of 
the basalt, which enabled it to force its way more readily between the basic 
sills, might proceed so far as partially or wholly to destroy these sills 
VOL.1I 2f 
