642 J. J. H. TEALL ON THE CHEMICAL AND 
of North America, which it so closely resembles in its petrological 
character and mode of occurrence. 
The object of this communication is to describe the chemical and 
microscopical characters of the rock of the Whin Sill. The obser- 
vations are based on specimens collected by myself, at many points 
in Durham, including Middleton, High Force, and Cauldron Snout; at 
a point about half a mile south of the locality marked Tyne Head on 
the Ordnance Map (Alston Sheet); and at Hot Bank, near Crag 
Lough, Bourgovicus, Bamburgh Castle, and many other points in 
Northumberland. Mr. Comozenski, the borough engineer of North 
Shields, kindly supplied me with specimens from Greenhead near 
Haltwhistle, Longhoughton and Little Mill near Alnwick, and 
Barrasford on the North Tyne. I have also examined the Little 
Whin Sill of Weardale, which occurs at a somewhat higher horizon 
than the Great Whin Sill of Teesdale. It is identical in its petro- 
logical characters with the latter rock. It will thus be seen that 
material has been collected from a sufficiently wide area to justify 
general conclusions as to the character of the rock, although it 
cannot be expected that all varieties have been observed. In no 
case are there any marked variations from the common type except 
such as depend, in all probability, on rapidity of cooling, and these 
may generally be found in one and the same locality. 
There is one aspect of the question to which I have paid scarcely 
any attention, although it is one of great interest and importance; I 
refer to the phenomena of contact-metamorphism. This metamor- 
phism is most marked in many cases, both at the upper and lower 
junctions. Limestones have become crystalline, and the shales have 
been converted into a kind of porcellanite, with development of 
garnet and other minerals. 
The subject has been referred to by Prof. Sedgwick, Mr. Clough, 
and other writers; but it would well repay further investigation, 
with both the microscope and the chemical balance *, 
PETROLOGICAL CHARACTERS. 
The rock is dark grey or bluish grey when freshly exposed. In 
texture it varies from compact to coarsely crystalline, the most 
common variety being one in which the individual constituents are 
just recognizable. by the naked eye. The compact variety occurs 
* The following papers contain important observations on the contact-meta- 
morphism produced by rocks similar in character to the Whin Sill :— 
Lossen. Metamorphische Schichten aus der palaozoischen Schichtenfolge 
des Ost Harzes. Z. D. G. G. 1869, p. 281. 
Ueber den Spilosit und Desmoisit Zinckens. Z. D. G. G. 1872, p. 701. 
Kayser. Ueber die Contactmetamorphosen der kornigen Diabase im Hartz, 
Z. D. G. G. 1870, p. 103. . 
Scuenck. Die Diabase des oberen Ruhrthals und ihre Contacterscheinungen. 
(Inaugural Dissertation.) Bonn, 1884. 
The work of these authors appears to establish the important fact that the 
metamorphism is accompanied by an actual transfer of felspar substance from 
the magma of the diabase to the surrounding rock. 
