574 ERUPTIVE AND METAMORPHIC ROCKS. 



sea-level ; the rock is a dark, close-grained, and often columnar rock ; it forms a 

 capping on the Coal-measures, and it was considered by Jukes to represent a lava- 

 flow during the Coal period. The Rowley Rag, as it is called, is largely quarried 

 for road-mending : similar rock occurs at Barrow Hill, two miles west of Dudley. 

 (vSee p. 1 88.) The hard grey dolerite of Pouk Hill, near Walsall, is more strikingly 

 columnar than that of Rowley. Titterstone Clee Hill in Shropshire is capped by 

 a mass of columnar dolerite ; and Knovvle Hill near Kinlet is formed of a mass of 

 dolerite overlying altered sandstone. These rocks and also basalt from Little 

 Wenlock and Arley Wood, near Shatterford, have been described by Mr. S. 

 All port. The Cornbrook Coal-field, as before mentioned, is to a large extent 

 covered by basalt, from 6o to 150 feet thick, and in some places the coal is 

 altered and 'sooty.' The rock is of considerable value as a paving-stone. (See 

 p. 190.) 



The intrusive diorites in the Warwickshire Coal-field closely follow the 

 bedding-planes of the strata ; ^ one of these rocks is known as the Griff-stone, 

 from Griff hollow, south of Chilvers Coton, near Nuneaton. The diorites pene- 

 trate the Hartshill quartzite and overlying Stockingford Shales between Ather- 

 stone and Nuneaton. (See p. 66.) Volcanic tuff (Andesites), with intrusions of 

 quartz-porphyry, and diabase, grouped as the Caldecote series, occur at Caldecote 

 Hill, north-west of Nuneaton : these beds are regarded as Arch^^an by Prof. 

 Lapworth. (See p. 45.) In the same district, further east, there are syenitic rocks 

 (quartz-syenite), etc., grouped as the Croft series, from Croft, between Leicester 

 and Hinckley. Syenitic rocks occur also at Huncote, Sapcote, Stoney Stanton, 

 and Potters Marston (Barrow Hill quarry), east of Hinckley. 



In Charnwood Forest many Igneous rocks are exposed. (See p. 45.) These 

 have been investigated by the Rev. E. Hill and Prof. T. G. Bonney, but they 

 speak of their age as uncertain, though some are possibly of the Old Red 

 Sandstone period. Most conspicuous among these rocks are the hornblendic 

 granites of Quorndon, Mount Sorrel ; and the Syenites of Groby, Bawdon Castle, 

 Markfield, Cliff Hill, Garendon, and Long cliff. Syenite has also been proved at 

 Baron Park, west of Leicester, and this appears to be intrusive. Bardon Hill is 

 formed of felstone. Outliers of the Charnwood Forest rocks occur at Enderby 

 (syenite and slate), Narborough (quartz-felsite), and at Croft Hill and other places 

 previously mentioned. '■* (.See p. 45.) 



Allusion has previously been made to the 'loadstone' of Derbyshire. (See 

 p. 159.) Near Matlock there are two bands of this contemporaneous basalt or 

 dolerite in the Carboniferous Limestone : they are pierced by the railway-tunnel 

 through High Tor, and are to be seen on the " Heights of Abraham." Mr. 

 Allport observes that the toadstone is an amygdaloidal basalt, regularly inter- 

 bedded with the Carboniferous Limestone, and was evidently a contemporaneous 

 lava-stream, partly scoriaceous, and partly of a compact basaltic structure. 

 Generally the cavities have been filled with carbonate of lime or other minerals, 

 thus forming an amygdaloid ; other specimens, however, have empty cavities, and 

 are as vesicular as recent lavas. ^ The beds average 60 or 70 feet in thickness, 

 and preserve their courses for many miles ; but they are often much decomposed. 



Basalt occurs in the Tortworth district, in Gloucestershire, at Charfield Green 

 and Damory Bridge, and at Crockley's farm the rock has yielded large geodes of 

 chalcedony and quartz, known as "buns." In .Somersetshire eruptive rocks occur 

 at several points in the Mendip Hills : at Middle Hope or Woodspring Hill, 

 at Swallow Cliff and Uphill railway-cutting (basalt or amygdaloidal dolerite), 



1 S. Allport, Q. J. XXXV. 637 ; A. Strahan, Brit. Assoc. 1S86, G. Mag. 1886, 

 p. 540 (with notes by F. Rutley), Geol. Surv. Map, 63 S.W., with revisions by 

 A. Strahan ; T. H. Waller, Brit. Assoc. 1886. 



2 Q. J. xxxiv. 225. Analyses of several of the rocks have been made by 

 E. E. Berry, Q. J. xxxviii. 197. See also W. J. Harrison, Midland Naturalist, 

 vii. 7, 41, and Geology of Leicestershire, etc., 1S77 ; S. Allport, G. Mag. 1879, 

 p. 481 ; Rev. T. N. Hutchinson, Rep. Rugby School Nat. Hist. Soc. for 1876, 

 p. 34. 



*> 3 G. Mag. 1870, p. 159 ; Q. J. xxx. 547 ; Jukes, G. Mag. 1S67, p. 468 ; Teall, 

 Brit. Petrogr. PI. ix. 



