ERUPTIVE AND METAMORPHIC ROCKS. 



569 



In the following pages the Igneous and INIetamorphic rocks are 

 described geographically rather than chronologically, for the precise 

 age of many of the Eruptive rocks has not yet been determined. 



The Cheviot Hills are formed partly of andesites and porphyrites (lavas and 

 tuffs) 1500 to 2000 feet thick, and partly of Sandstones belonging to the Lower 

 and Upper Old Red Sandstone. (See pp. 121, 143, and Fig. 16, p. 114.) The 

 chief focus of eruption appears to have been near Cheviot, and the beds were 



Fig. 96. — Section through Ward's Hill, near Rothbury. 



(W. Topley.) 



AV.N.W. ^ B ,r7--7>>^ E.S.E. 



-W.T 



L. Great Limestone. 



B. WTiin Sill. 



mostly subaqueous. The volcanic rocks which form much of the higher ground 

 are of Lower Old Red Sandstone age. There are also intrusive rocks, quartz- 

 felsites, and augite-biotite-granites.^ Melaphyre or Dolerite occurs at Stichill 

 among the Tuedian Beds. 



The basaltic rocks of the north of England occur either as dykes, cutting verti- 

 cally through the sedimentary rocks, or as beds or sheets lying among them. 

 The term " Whin" is locally applied to rocks hard enough to be used for road- 

 metal, and the term "Sill" is applied to rocks that occur in layers more or less 

 parallel with the associated beds. 



The Great Whin Sill, which forms so striking a feature on geological maps, is a 

 mass of intruded diabase, which occupies various positions in the Carboniferous 

 Limestone and Yoredale rocks in Northumberland, Cumberland, and Durham. 

 Messrs. Topley and Lebour observe that this rock is best known in Teesdale, 

 especially in the two fine waterfalls of High Force and Cauldron Snout. It also 

 appears along the face of the Pennine escarpment, and is beautifully exposed in the 

 " Nicks " which furrow the face of that range. It is sometimes only twenty-four 

 feet in thickness, but it is as much as 120 feet at Alston Moor, and over 200 feet in 

 Teesdale (see Figs. 96, 97) ; it frequently alters the beds above it as well as those 

 below, and in certain places portions of the sedimentary rocks have been assimi- 

 lated. Its exact age is as yet undetermined ; it may have been formed at the close 

 of the Carboniferous period.^ Where easily accessible, the whinstone is used as 

 road-metal ; its decomposition gives rise to a ferruginous clay. 



The name " Harkess Rocks " is given to the low-lying foreshore between the 

 Bamburgh boat-house and Budle Point, in Northumberland ; and there the rocks 

 consist of whinstone, sandstone, shale and limestone. The greater part of the 

 eruptive rock belongs to the Great Whin Sill, and is directly connected with the 

 range of basaltic outbursts of which the Spindlestone Crags, the rock on which 

 Bamburgh Castle stands, and the Fame Islands form part.^ The Beadnell basaltic 



^ J. J. H. Teall, G. Mag. 1883, pp. 100, 145, 252 ; Johannes Petersen, /l>id. 

 1884, p. 226; A. Geikie, Trans. Edin. Geol. Soc. ii. ; J. Geikie, Good Words, 

 1876. 



2 Topley and Lebour, Q. J. xxxiii. 406 ; C. T. Clough, G. Mag. 18S0, p. 433 ; 

 Q.J. xxxii. 466. See also T. G. Bonney, P. Geol. Assoc, vii. 104; J. J. H. 

 Teall, Q. J. xl. 640, British Petrography, Plates xii. xiii. ; Sedgwick, Trans. 

 Cambr. Phil. Soc. 1824. 



* G. A. Lebour and M. Fryar, Proc. N. of Eng. Inst. Mining Engineers, 

 vol, xxvi. 



