W. M. HiitcJiings — Roclcs of Great Whin Sill. 71 



tlie other side of it. If there be recrystallized quartz, this also often 

 comes quite sharply up to the contact-line. 



In some instances there does appear to be a very narrow streak of 

 more indefinite matter, possibly denoting interfusion ; and there are 

 other cases where a very noticeable band is seen of what has clearly 

 been of a tachylitic nature; though whether we ought to regard it 

 as a true tachylite, — i.e. a product simply of rapid cooling of the 

 edge of the molten igneous rock, — or whether it is moi-'e a result of 

 interfusion, I think cannot be settled, because chemical analysis 

 would not here give a sufficiently definite answer. So far as 

 microscopical evidence can help us, I rather incline to the view that 

 it points to interfusion having taken place to some extent. Thus, 

 the most striking example is one from Middleton Wood, near 

 Belford. In the hand-specimen the tachylitic material was some 

 two inches thick, and in the slide prepared from it, over the line of 

 contact, there is nearly half an inch of it. The limestone is simply 

 crystallized, as is also silica which it contained. It has not had any 

 new minerals formed in it, but quite close to the junction there are 

 a few colourless garnets, just a narrow string of small crystals and 

 grains. Then conies the tachylitic band, mainly a yellow to red- 

 brown glass, with a good deal of indefinite, speckly, felsitic-looking 

 matter, and chloritic decomposition products, but with some felspars 

 and augites of good size dispersed in it, and a few prisms of 

 enstatite. With these is also a good deal of garnet in small grains, 

 and at some parts patches of it of much larger size. As no garnet 

 occurs in the altei'ed limestone except at the actual contact, and as 

 it occurs in the tachylitic band, it seems likely to be a product of 

 the interaction of limestone and Whin. No garnets seem ever to 

 occur in the normal Whin Sill rock. 



In some other specimens examined there is, again, a narrow band 

 of what appears to be another product of such interaction. There 

 is a seam of what appears to have been tachylitic, now very much 

 obscured by calcite and chlorite due to decomposition. The lime- 

 stone is perfectly free from any new mineral formation. But on the 

 side towai'ds the Whin comes a zone of close-grained igneous rock, 

 a narrow strip of which is coloured brownish-red of a peculiar 

 shade. Under higher powers it can be made out that this reddish 

 band contains swarms of minute flakes of mica, and that it is from 

 these that it derives its colour. Here and there the compact swarms 

 of this mica open out, and become more scattered and larger in size, 

 some individuals being seen as fairly well-bounded crystals, which 

 can be recognized as a deep brown-red very dichroic biotite, some 

 of the best flakes giving a good optic figure in convergent light 

 with "iV inch objective. None of this mica is ever seen in the 

 normal Whin, and 1 have not seen any signs of it except at contacts 

 with pure limestones. It certainly appears to be an endomorphic 

 formation in the igneous rock, brought about by interaction with the 

 limestone, though it does not seem easy to explain the chemical re- 

 actions which have been concerned in it. I do not recollect any 

 mention of a similar result on an intruded igneous rock, and have 

 Certainly never seen it myself in any other case. 



