207 



I. C'eveland Dyke, Great Ayton, Yorkshire (STOCK). 



II. Armathwaite Dyke, Armathwaite, Cumberland (STOCK). The Armathwaite Dyke is a 

 continuation of the Cleveland Dyke. The two specimens analysed by Mr. STOCK were taken 

 from points about sixty miles apart. 



III. Acklington Dyke, Ackliiigton, Northumberland (STEAD). 



IV. Tynemouth Dyke, Tynemouth, Northumberland (STEAD). 

 V. Morpeth Dyke, Morpeth, Northumberland (STEAD). 



VI. HettDyke (BELL). (1) 



VII. Intrusive sheet in Coal-measures, near Hett Dyke (BELL). 



VIII. South Dyke, High Green, near Bellingham, Northumberland (STEAD). 



The most important mass of basic igneous rock in the north of 

 England is that known as the Whin Sill. The relations of this mass to 

 the strata of Lower Carboniferous age have given rise to some discussion 

 but it is now generally recognized as being intrusive. (2) It is exposed 

 as an inlier in Teesdale, and may be followed up the valley for a con- 

 siderable distance. It reappears in the Cross Fell escarpment in Cumber- 

 land where its intrusive character is easily recognized, and it may be 

 traced thence with slight interruptions across the county of Northumber- 

 land to the sea coast at Dunstanburgh, following in a general way the 

 strike of the beds with which it is associated. As the strike bends round 

 to the N.W. in the northern portion of Northumberland, the Whin Sill re- 

 appears on the coast at Bamborough, and may be traced from this point to 

 Kyloe, where it is last seen. The distance from the most southerly 

 point in the Cross Fell escarpment where the Whin Sill is exposed to 

 Dunstanburgh is sixty or seventy miles, so that the total mass of the 

 Whin Sill must be enormous. It forms an intrusive sheet which 

 varies in thickness from 20 to 150 feet. Its average thickness in 

 Northumberland is about 80 or rOO feet, according to Professor 

 LEBOUR. The date of its formation is not known with precision. It 

 must be posterior to the Lower Carboniferous period and anterior to 

 the disturbances which have affected the district. This seems to 

 imply that it is post-Carboniferous and pre Permian. 



The prevailing type of rock is a distinctly crystalline (medium 

 grained) mixture of plagioclase, pyroxene, and titaniferous iron ore. 

 Olivine does not appear to occur, but its place is taken in certain 

 varieties by a rhombic pyroxene. Where the rock bulks very largely, 

 as at Tyne Head and Cauldron Snout, we find irregular masses of a 

 coarsely crystalline variety. In these the augite is frequently developed 

 in the form of long, blacled crystals. The individuals are somewhat 

 irregular in outline, but the dominant forms are easily made out to be 

 the two vertical pinacoids (100) and (010), the prismatic faces being 

 simply represented by truncations. Each individual is, as a rule, twinned 

 according to the ordinary law, (see Fig. 21, page 157), and several in- 

 dividuals are frequently seen to be arranged parallel to each other in 

 the rock. The direction in which the crystals are elongated is that of 



(1) Proc. Roy. Soc., 1875, p. 543. 



(2) Messrs. TOPLKY & LEBOUR. On the intrusive character of the Whin Sill of North- 

 umberland. Q. J. G. S., Vol. XXXIII., p. 406., 1877. 



