ERUPTIVE AND METAMORPHIC ROCKS, 5/1 



Walker, where it enters Northumberland. It was formerly quarried north of the 

 Boldon Hills : it is about 44 feet thick in places, and is not known to penetrate 

 any formation later than the Coal-measures. The Tynemouth Dyke is exposed at 

 the base of the Castle Rock, Tynemouth : it is 10 feet wide, and cuts the Coal- 

 measures. The Coley-hill Dyke near Newcastle is possibly connected with that 

 at Tynemouth : the rock was formerly worked for road-metal. Another, termed 

 the Brunton Dyke, occurs near Whitfield in West Allendale. Several parallel 

 dykes known as the Seaton and Hartley Dykes may be seen on the shore between 

 those localities, and also inland at Shankhouse collieries : these rocks are quarried 

 for road-metal. The Morpeth Dyke crosses the Wansbeck near Morpeth. 



Mr. S. Allport has described the rocks, formerly called "Greenstone," that 

 constitute the axis of the ridge of which Ercal Hill and the Wrekin form 

 the greater portion. They comprise an extensive series of regularly stratified 

 agglomerates and ashes, alternating with masses of altered pitchstones or felsites. 

 At Lea Rock he obtained interestmg examples of ancient spherulitic pitchstones 

 (Rhyolites) ; and these he considered were originally identical with some of the 

 glassy volcanic rocks ejected during recent geological periods.' The Wrekin 

 Volcanic series (including the Lilleshall beds) is grouped by Dr. Callaway as 

 Archaean.- (See p. 44.) 



There are many volcanic rocks in the Lake District, some of which were 

 investigated by the Rev. J. Clifton Ward.^ Sir A. C. Ramsay stated his belief 

 that the Cumbrian volcanoes were mainly subaerial, since some 12,000 feet of ash- 

 and lava-beds have been accumulated in Cambrian times (Volcanic series of 

 Borrowdale), without any admixture of ordinary sedimentary material, except quite 

 at the base. Mr. Ward believed also that one of the chief volcanic centres of the 

 district had been close to the present site of Keswick ; the low craggy hill called 

 Castle Head, formed of diabase, representing the denuded stump or plug of an old 

 volcano, which may have been as large as Etna. Another vent may have been at 

 Friar's Crag, by Derwentwater. On Eycott Hill, north-east of Keswick, much 

 porphyritic lava (dolerite) is developed, with but little ash, and the beds attain 

 a thickness of nearly 3000 feet. In these beds Prof. Bonney has observed a 

 variety of Enstatite.'* Mr. Ward has observed that the metamorphism among the 

 Cumbrian rocks increases in amount as the great granitic centres are approached ; 

 and that it took place mainly at the commencement of the Old Red Sandstone 

 period, when the rocks must have been buried many thousands of feet deep. 

 Thus, the Skiddaw Slate, on approaching the granite of the Caldew valley or 

 Skiddaw forest, passes into chiastolite slate, and this again into a rock of a schistose 

 character (miscalled Hornblende slate), which has been used for making sets of 

 musical stones, several of which may be seen in Keswick. The schist (or spotted - 

 schist) passes into Mica-schist : hence there is a complete passage in the field 

 from unaltered clay-slate, through chiastolite slate and spotted schist, to mica- 

 schist. The junction between mica-schist and granite is well defined, but the 

 latter sends out veins into the adjoining rocks. The granite is of a grey colour, 

 cpnsisting of white felspar, dark mica and quartz. (See pp. 78, 80.) 



The Criffel granite in Kirkcudbrightshire is a rock of which fragments occur in the 

 Drift of the North of England and North Wales ; it contains a good deal of black 

 mica.* The Eskdale granite is coarse-grained with pale felspar, often tinged of 

 a reddish hue. At Wastdale Head it occurs over a small tract at the foot of Kirk 

 Fell and Lingmell, probably connected with the main mass of Eskdale granite, 

 along the axis of W^astwater. The granite of Dufton is seen at Banky Close, on 

 the north-west flank of Dufton Pike. A small granitic mass has also been worked 

 at Holebeck, Black Comb. 



The granite of Wastdale Crag and Wastdale Pike, south of Shap, consists of 

 a ground-mass of quartz, felspar, and black mica, porphyritically enclosing large 



1 Q. J. xxxiii. 449. 



^ Q.J. xxxvi. 536; xxxviii. 1 19; xlii. 481. 



2 Geology of the northern part of the English Lake District, 1S76 ; Q. J. xxxi. 

 405, 568, xxxii. 2, 7 ; Goodchild, P. Geol. Assoc, ix. 471. 



* G. Mag. 18S5, p. 76. 



* See D. Mackintosh, Q. J. xxxv. 425. 



