SOME NORTH-OF-ENGLAND DYKES. 2138 
that is for a distance of 1000 miles, the same composition and 
structure. In a future paper I hope to give an account of the 
Whinsill, when these points will be more fully discussed. 
I now proceed to describe the rock of the Cleveland, Cockfield, 
and Armathwaite dyke. The facts recorded are based on the ob- 
servation of numerous specimens collected by myself at Ayton, 
Preston, Bolam, Cockfield, Bowles Beck, Tyne Head, and Arma- 
thwaite. Many sections* have been prepared of the rocks from each 
of these localities, and several chemical analyses have been placed 
at my disposal by my friends Messrs. Stead and Stock. I have also 
made a large series of specific-gravity determinations. All these 
observations prove the essential uniformity of the rock along the 
whole line of outcrop. 
Petrology of the Cockfield and Armathwaite Dyke. 
The rock is usually of a dark grey or bluish grey colour when 
freshly broken, butit becomes darker after exposure to the air. Itis 
porphyritic in texture, and frequently, though not invariably, 
possesses a subconchoidal fracture. The porphyritic crystals are 
glassy-looking felspars, which occasionally show on their cleavage- 
faces the striations due to repeated twinning. ‘The crystals are 
sometimes whole, sometimes fragmentary, the fragmentary condition 
evidently indicating that they were not developed where we now 
see them. Sections more or less parallel to the basal plane are 
usually long in proportion to their width; whereas those which are 
parallel to the brachypinakoid have tolerably uniform dimensions in 
the different directions. These facts doubtless show that the crystals 
are less developed in the direction of the macro-diagonal than 
in the directions of the other two crystal axes. The sections, when 
measured across, vary from 1 mm. to 5 mm. These porphyritic 
erystals are distributed throughout the ground-mass with consider- 
able regularity along the whole length of the dyke. ‘Three or four 
conspicuous ones usually occur on a square inch of surface, thus 
giving the rock of this dyke a definite macroscopic character, by 
means of which it can be distinguished from that of any other dyke 
known to me in the north of England. The ground-mass varies 
according as the specimen is taken from the margin or from the 
centre of the dyke. At the margin and for a few inches inwards 
it appears to the unaided eye compact and homogeneous, and the 
large porphyritic crystals, which are just as well developed here as 
in the centre, stand out very conspicuously ; beyond this, however, 
it becomes finely and, in a few cases, where the dyke is thick, even 
coarsely crystalline. 
The rock, especially at the margins of the dyke, frequently 
effervesces freely with hydrochloric acid; this indicates the deve- 
lopment of carbonates by secondary processes, and marks the first 
stage of decomposition. A further stage may be well seen at the 
* T have had 27 sections prepared from my own specimens, and several others 
have been placed at my disposal by my friends Messrs. I’Anson and Trechmann. 
