C. T. Clough—The Whin Sill of Teesdale. 445 
of form. When it is in the form of a dyke, or of a boss, we cannot 
see so clearly what the physical relations between it and the 
surrounding beds are as we can when it is in the form of a sheet, for 
it is only in the latter case that we have our sections inclosed 
between definite and recognizable horizons—between a top and a 
bottom. Yet there are not wanting indications of assimilating 
power derived directly from dykes, as is shown by the following 
remarks made by Professor Hughes on the mica trap dykes of the 
neighbourhood of Kirkby Lonsdale and Sedbergh (Geol. Survey 
Memoir, 98 8.E., pp. 41-42):—‘ They do not appear to displace but 
to replace portions of the rock in a manner that would suggest 
that portions of the sedimentary rock had been assimilated to the 
mica trap. The edges of the beds are not broken off or pushed aside 
as one might expect would be the case were a viscid molten mass 
squirted violently into the sedimentary rocks, and the opposite walls 
of the dyke often do not correspond as they should do if the molten 
mass quietly filled an open fissure, and moreover, when the rocks 
can be examined in the neighbourhood of the dykes, such fissures do 
not appear. It may be worth consideration. whether in some cases 
it might not be possible that the action of gases or of hot water 
holding minerals in solution, communicating along lines of fissures 
with the joints, might produce the phenomena observed.” 
And I think it is scarcely necessary to add that many large granitic 
or syenitic masses, even though sharp junctions are commonly found 
round their margins, show still more striking instances of absence 
of disturbance. 
APPENDIX. 
I have thought it worth while to make a rough estimate of the 
kind of mixture we should get if we imagined the rocks which occur 
in the neighbourhood of the Whin Sill in Teesdale to be melted 
down in about their natural proportions. 
A very common position for the Whin is about 20ft. below what 
is called the Single Post Limestone,’ and I think that we might take 
the 200ft. of beds which usually come below this horizon as a fair 
type of the rocks among which the Whin occurs generally. This 
200ft. is made up something as below, beginning from the top :— 
SLES dadebatiiiectpen SAS has oh aes, scat fied bahia aria amis bl 8 
Limestone (Tyne Bottom) ... 0... 0... we ae eee BDF. 
Marhly Sandstqnet pe eye ase! see Geek) pew BROMUS 
EP ERESLONG) <1, ah apes ere MA Taare CORE eke dans dence 
SUE, fae saa pp) dlenphe repay HOM GNGS > «43 lecaiT abr ase se% Aas 
RGRUSUONG, (5) wie teen Sau Gen tit nen, Mceekireeed Weta 
MBIMGSEONG js: spp) saeht eee tee ate’ (EOLIR canto uke) Chex SEED 
Shale and Sandstone in about equal proportions ... ... 30ft. 
Limestone ; Tit. 
Or we might say that we have roughly in this 200ft., TOft. of 
limestone, 7Oft. of sandstone, and 60ft. of shale. 
To show the composition of the limestone a specimen of the Tyne 
* Curiously enough it is never once seen in the Dale immediately below the Tyne 
Bottom Limestone, although this is the position ascribed to it by those who advocate 
its contemporaneous character, 
