128 
THE TERTIARY VOLCANOES 
BOOK VIII 
of a dyke consolidated first, and are therefore finest in texture. For the 
most part, each dyke appears to be due to a single uprise of molten matter, 
though considerable movements may have taken place within its mass before 
the whole stiffened into stone. Some particulars regarding these movements 
will he given in section 12 of the next Chapter. It has already been men- 
tioned that in large dykes which have served as volcanic pipes, it is con- 
ceivable that while the material next the outside consolidated and adhered 
to the walls, the central portion may have remained liquid, and may even 
have been propelled upward and have been succeeded by a different kind of 
magma, as has been suggested by Mr. Iddings. In such cases, which, it 
they occur, are probably excessively rare, we may expect that the earlier and 
later material will not be sharply marked off from each other, unless we 
suppose that the whole of the earlier liquid magma was so entirely ejected 
that only its congealed marginal selvage was left as bounding walls for the 
newer injection. 
Where, after more or less complete consolidation had taken place, the 
fissure opened again, or from any other cause the dyke was split along its 
centre, any lava which rose up the rent would tend to. take a finer grain 
than the material of the rest of the dyke, and might even solidify as glass. 
Large scattered crystals of felspar, of an earlier consolidation than that 
of the minuter forms of the same mineral in the general groundmass of the 
rock, give a porphyritic structure and andesitic character to many dykes. 
Occasionally such crystals attain a considerable size. Mr. Clough lias 
observed them in some of the Argyleshire dykes reaching a length of 
between three and four inches, with a thickness of two inches. Sometimes 
they are distributed with tolerable uniformity through the substance of the 
dyke. But not infrequently they may be observed in more or less definite 
bands parallel with the boundary walls. Unlike the younger lath-shaped 
and much smaller felspars of the groundmass, they show no diminution 
either in size or abundance towards the edge of the dyke. On the contrary, 
as already mentioned, they are often conspicuous in the close-grained 
marginal strip, and may be found even in the glassy selvage, or touching 
the very wall of the fissure. Indeed, they are sometimes more abundant in 
the outer than in the inner portions of a dyke, having travelled outwards to 
the surfaces of earliest cooling and crystallization. 
Mr. Clough has given me the details of an interesting case of this kind 
observed by him in Glen Tarsan, Eastern Argyleshire : — “ For an inch or so 
from the edge of this dyke,” he remarks, “porphyritic felspars giving 
squarish sections, and ranging up to one-tliird ol an inch in length, are so 
abundant as nearly to equal in hulk the surrounding groundmass. lor the 
next inch and a half, they are decidedly fewer, occupying perhaps hardly an 
eighth of the area exposed. Then for a breadth of three inches they come 
in again nearly as abundantly as at the sides ; after which they diminish 
through a band 27 inches broad, where they may form from to T V of the 
rock.” He found another case where, in a dyke several yards wide, por- 
phyritic felspars, sometimes an inch long, are common along the eastern 
