624 KEPORT— 1886. 



5. Gil the Bochs surrounding the Wanvickshire Coalfield, and on the Base 

 of the Goal-measures} By Aubrbt Strahan, M.A., F.G.S. With an 

 Afpendix on the Igneous Rocks of the Neighbourhood, by P. Rutley, 



F.G.S. 



(By permission of the Director-General of the Geological Survey.) 



The discovery by Professor Lapworth of fossils of Lower Silurian age (Cambrian, 

 Sedg.) in some shales underlying the productive coal-measures of Warwickshire has 

 proved that the determination by the Survey of these shales and of some underlying 

 quartzite as Lower Coal-measures and Millstone Grit respectively was erroneous. The 

 beds had been (previous to the Survey) described by the Rev. J. Yates (' Trans. Geol. 

 Soc' Ser. 2, vol. ii. p. 237) as Silurian, but in consequence of their apparent con- 

 formity to the productive measures, and in the absence of fossil evidence, were 

 subsequently identified as above, in spite of a correct view as to their age being 

 held by Professor Jukes. A re-examination of the district has proved that (1) the 

 conformability of the Coal-measures with the underlying shales is apparent only, 

 and not real ; (2) the Coal-measures are based by an impersisteut sandstone con- 

 taining pebbles, and resting on the denuded surface of the Lower Silurian rocks ; 

 (3) the intrusion of certain igneous rocks (diorites, &c.) in the older series was 

 entirely pre-Carboniferous. 



The oldest rocks seen are the Caldecote Igneous Series, which rise from beneath 

 the quartzite between Nuneaton and Hartshill. This series consists of a finely 

 laminated rock, probably a tuff", with intrusions of diabase and quartz-porphyry. 



Upon this series rests the Hartshill quartzite, with a well-bedded conglomerate 

 at its base, containing fragments of the Caldecote Series. Upwards this quartzite 

 passes conformably into a thick mass of shales, red in the lower, and grey or black 

 in the upper part. In these shales (the Stockingford Shales of Professor Lapworth) 

 have been found fossils, proving them to be of Lingula Flag, and perhaps in part 

 of Tremadoc age. The shales and quartzite are traversed by sheets of diorite, &c., 

 which generally follow the bedding very closely, but are known to be intrusive by 

 the alteration they have eflPected upon the shales above, as well as below them, and 

 by their occasionally breaking across the beds. 



The base of the Coal-measures has been proved in a colliery at Hawkesbury, 

 and is seen in the railway-cutting at Chih-ers Cotou, where a fire-clay, based by a 

 few inches of sandstone, is seen resting on the Stockingford Shales. Further north 

 the workable seams rest almost directly on these shales, but near Oldbury a thick 

 pebbly sandstone, resting with a marked unconformity on shales and diorites, 

 forms the base of the Coal-measures. Fm-ther on this sandstone thins out again, 

 but reappears with its former character north of Merevale. A similar relation is 

 found at Dosthill, where, however, the want of parallelism between the Carboni- 

 ferous and Silurian strata is very conspicuous. The impersistent sandstone, forming 

 the base of the Coal-measures here, is coarsely conglomeratic. The descriptions of 

 the base of the Coal-measures of South Staifordshire (Jukes, ' Geol. Survey Memoir') 

 is applicable almost word for word to the Warwickshire localities. 



Towards the east the Lower Silurian rocks are overlain by the New Red Marl 

 and Waterstones, the actual boundary being formed for a large part of the distance 

 by a fault, which has been proved in coal-workings at Polesworth, and is seen in 

 Merevale Park. This has been wrongly described (' Coal Commission Report,' 1879, 

 vol. ii. p. 494) as ' one of the grandest lines of fault that can be seen anywhere ' ; 

 it is, on the contrary, a fault of very small importance, especially towards the 

 south. Tlie Trias is seen in four instances to rest naturally on the old rocks, while 

 the same relation has been proved in several borings made in the plain of New Red 

 Marl of West Leicestershire. The result of these observations is to show that it 

 would be useless to search for Coal-measures beneath the Trias over the part of this 

 plain lying south of Market Bosworth, the Carboniferous rocks having either never 



' Published in extenso in the Geol. Mag. Dec. 3, 1886, vol. iii. p. 540. 



