444 C. T. Clough—The Whin Sill of Teesdale. 
Tf the altered rock had been in its intimate composition of com- 
plicated chemical character, then, under the influence of the basalt’s 
heat, its particles might have entered into fresh chemical com- 
binations among themselves, and have assumed an appearance 
slightly resembling that of the basalt, and this resemblance might 
have been increased by the vapours ascending from the basalt. But 
the altered rocks in Teesdale are generally of comparatively simple 
character, are either good siliceous sandstones or fairly pure lime- 
stones, etc., and in their case the alteration by heat could have given 
rise to nothing greatly different in appearance from their unaltered 
representatives. 
There are many quartz veins in the Dale which there can be no 
doubt have been formed in great part not simply by the growth of 
quartz in an open fissure, but also by the replacement of limestone 
by quartz along the sides of such a fissure. This is shown by the 
quartz vein stuff sometimes containing casts of crinoids, corals, ete. 
In other veins the ironstone contains similar casts, and must there- 
fore have been formed by the replacement of limestone by the 
carbonate or peroxide of iron. And yet in both these cases the 
passage between the limestone which has been acted on and that 
which has not is sometimes so sharp—being comprised within an 
inch or so—that it may help us to conceive how igneous rocks may 
have sharp boundaries, and may still have been formed in part by 
the alteration of the beds whose place they occupy. I do not mean, 
of course, that these junctions are so sharp as those of the Whin, or 
that in other respects the cases are quite parallel, but, as far as 
regards our present consideration, the main differences between them 
are, I think, that the substances which replaced the limestone at 
once became solid and took up a fixed position at the moment they 
replaced it, while in the case of the igneous rock the replaced and 
the replacing substance continued to form together a fluid mass. 
I am conscious that the explanations I have attempted of these 
difficulties are only very crude, and I must leave it to others to 
find out more satisfactory ones. I fully admit the sharpness of the 
junctions and the general uniformity of composition of the Whin as 
much as any one. The question is not as to their existence, but as 
to their value. And in settling this it seems to me that the broad 
physical evidence in the field is of paramount importance. Whatever 
this evidence clearly points to we ought to accept. There may be 
difficulties facing us, but if we have made quite certain of our facts 
to begin with, we may be sure that these difficulties are only 
apparent, and will in time be cleared away. 
If it be granted that there is nothing impossible or unusual in 
these sheets of basalt eating up the beds among which they 
have been thrust, it will be necessary also to allow this power to 
various other intrusive masses. The same intrusive mass will, 
according to the circumstances surrounding it, sometimes take the 
form of a sheet and sometimes of a boss and sometimes of a dyke. 
There can be little doubt that it will continue to have essentially the 
Same properties and powers through all these comparative accidents 
