C. T. Clough—The Whin Sill of Teesdale, 443 
_If we grant the probability of such a general circulation, and if 
we consider the immense area over which the Whin Sill is known 
to extend, and the still larger area over which it may be supposed to 
extend, both laterally and still more vertically, in connexion with 
some deep-seated mass, it does not seem a very likely thing that 
these devoured beds, when their material is distributed through the 
intrusive mass, should have much effect in altering its original 
composition. For large though these beds may, in some places, be, 
yet we must consider that they are not always of the same chemical 
composition ; that sometimes they are of limestone, and sometimes 
of sandstone, and sometimes of shale, and sometimes of other rocks, 
and that it may well be that the sum total of the devoured beds 
when boiled down would produce a mass not much unlike that of 
basalt. For if the sedimentary beds are but the “ruins” of a 
primitive igneous mass, we must suppose that an average of them 
would represent fairly well the chemical constitution of this mass.’ 
In a country where the beds among which the Whin lies are not 
very dissimilar to it in composition, and would of themselves make 
a rather fusible mixture, it would not be unnatural to suppose that 
there would be a greater tendency for them to be assimilated than in 
other countries; and this may be the reason why in Teesdale there 
seem to be such prominent cases of missing beds. 
It may be objected in the second place that there is no trace of any 
passage in character between the Whin and the other beds, but that if 
it is true that these beds have been at times completely boiled down 
into Whin, we should expect frequent instances of their gradual 
passage into Whin. As a rule nothing could be sharper than the 
junction between the Whin-and the sedimentary beds: it is like a 
knife edge. Near the Winch Bridge, and at a few other places, the 
junction specimens seem to contain irregularly alternating bands of 
Whin and of these beds, and it is not always easy tu say which is 
which; but this is decidedly exceptional. This sharpness of 
junction is, however, to be seen in the results of nearly all chemical 
experiments, and is, I think, to be explained by the definiteness of 
character of chemical action.” Some molecules of SiO, or of CaCO,, ete., 
have been absorbed by the basalt: these, we may suppose, have 
taken part in chemical reactions with certain constituents of the 
basalt, and have thus lost all outward likeness to their former 
selves. Other molecules have not combined chemically with any 
part of the basalt, but have only been induced by its heat to build 
themselves up in slightly different shapesfrom what they had before, 
and these still retain to a recognizable extent the chief of their old 
characters. And between these two sets of molecules a sharp dis- 
tinction has thus been set up. 
1 Tt is true that if we simply regard the more ordinary sedimentary rocks, lime- 
stones, sandstones, and shales, we should find in them, when compared with most 
igneous rocks, a decided deficiency in the alkalies. But we have also to call to mind 
the great beds of Rock Salt (NaCl), Carnallite (KCl, MgCl,, 6 H,O), ete., which 
exist in many places. 
2 The late Rev. J. C. Ward has already, in reference to the Eskdale and Shap 
Granites, raised the question whether an altered rock may not haye a sharply- 
defined margin (Q.J.G.S8, vol. xxxi. No, 124, p. 592). 
