Nuneaton, Warwickshire. 153 



Allport. Watts ' calls attention to the lamprophyric aspect of the 

 rock, which he terras a camptonite ; and with this description 

 Harker 2 agrees. Hatch 3 admits the lamprophyric features and 

 incidentally refers to the lack of recorded analyses. 



The conclusions arrived at in this paper are hased upon a detailed 

 examination of the rock in situ, upon a microscopic investigation 

 of a large number of thin sections, and upon a partial chemical 

 analysis of the prevalent rock type. 



2. The Intrusive Character of the Marston Jabet Hock. 



There is no question as to the intrusive character of the rock ; 

 for completeness, however, the evidence for intrusion is set out below 

 in some detail, the features described being those observed at the 

 quarry in August and December, 1912. 



The country rock is Coal-measure shale horizontally bedded. In 

 the middle of the quarry is a horizontal platform of shale from above 

 which igneous rock has been removed. At the quarry face the 

 latter rock extends for several feet below the plane of the middle 

 platform ; it is therefore clearly transgressive. 



At the north-east end the quarry face is a pillar of laminated shale 

 continuous with the platform below and extending upwards to the 

 ground level. The shale is injected with narrow veins and strings 

 of ' white trap ' along bedding- and joint-planes. The veins and 

 strings are frequently Jransgressive, and may also be traced into the 

 main mass of igneous rock. 



To the east and south long slabs and smaller lenticular masses of 

 shale are seen embedded in the igneous rock. In the southern face 

 two constant horizons of igneous rock are separated by a shale band 

 2 feet thick ; both the shales and the igneous rock sag slightly. 



The shale bordering the tramway incline is baked a brick red for 

 a distance of about 2 feet above the igneous rock. Shale at the 

 contact is usually porcellanized for a distance of one inch, and 

 indurated for a greater, though variable, distance beyond. Lit-par-lit 

 injection of the shale is frequent. The texture of the igneous rock 

 near its junction with the shales is compact and basaltic, whereas in 

 the centre of the mass it is crystalline. 



The rock is obviously a sill-like intrusion in the shale, which, 

 however, appears to have suffered little or no disturbance. 



3. Macroscopical Features of the several varieties of the Intrusive Rocl\ 

 The prevalent rock is greenish- grey crystalline, of fine or medium 

 texture, having a distinctly dappled or mottled appearance when 

 wet, owing to concentrations of hornblende and of felspar as minute 

 but discrete clots — glomeroporphyritic structure, which is assumed 

 by both hornblende and felspar. Pyrites in specks and irregular 

 strings is very abundant. 



The mottled variety passes insensibly into a more homogeneous, 

 finely crystalline rock, with the aspect of the Nuneaton and Ather- 

 stone 'diorites'. Both the varieties noticed effervesce slightly on 



1 Proc. Geol. Assoc, vol. xv, pp. 394-96, 1897. 



2 Petrology for Students, 1908 ed., p. 157. 



3 Text Book of Petrology, 1909 ed., pp. 329-30. 



