GEOLOGY 



sand deposits were succeeded by the finer mud of the Stockingford 

 Shales, although the waters must have become sufficiently clear and 

 calm at one time to have allowed of the formation of the Hyolite 

 Limestone from the remains of various mollusca. The only fossils in 

 the lower two divisions of the Quartzite are a few worm-burrows, sug- 

 gestive of the sands having been deposited along a shore ; the Shales 

 however as we have seen contain abundant evidence that the Cambrian 

 seas were peopled with a considerable fauna. 



Intrusive Igneous Rocks. The volcanic activity which is evidenced 

 by the igneous origin of the Caldecote rocks probably continued or 

 was reopened probably in immediately post-Cambrian time ; for both 

 the Hartshill Quartzite and especially the Stockingford Shales are 

 traversed by many sills and dykes of diorite (camptonite), which are 

 evidently solidified masses of molten rock forced up from below into the 

 Cambrian sediments. There is no evidence whatever to show that these 

 ever reached the surface and produced volcanoes, terrestrial or submarine. 

 The sills and dykes generally follow the bedding, but frequently cut 

 through the strata, baking and altering them. Yates perceived their 

 intrusive character in 1824. Allport 1 gave a figure of a section showing 

 this at Chilvers Colon railway cutting. Mr. Fox-Strangways 2 mentions 

 that in the quarry south of Merevale church the Stockingford Shales 

 dip at 1 5 to the south-west, while the igneous rock inclines at an angle 

 of 35 in the same direction. 



The sheets of diorite vary from mere threads less than a foot thick 

 to masses over a hundred feet through. They attain a great develop- 

 ment in Merevale Park and at Chilvers Coton. They have been 

 wrought for paving-cubes. One of the sills is well exposed in the 

 Midland Railway Company's quarry at Nuneaton station ; the jointing 

 of the rock is at right angles to the quartzite beds between which it was 

 intruded and cooled. At the entrance to Messrs. Tyre's quarry a thin 

 sheet of diorite intruded into the lower layers of the quartzite has segre- 

 gated on cooling into basic clots and acid veins. 



The microscopic structure and composition of these igneous rocks 

 have been described by Allport, Waller, Teall, and Watts ; it was 

 Allport's recognition of the fact that these rocks differed from the Car- 

 boniferous dolerites which gave an early hint that the Stockingford 

 Shales were no part of the Coal Measures. They consist essentially 

 of a triclinic felspar and hornblende, with some magnetite and apatite. 

 Augite and olivine are sometimes present ; "and Professor Watts 3 remarks 

 that the rocks would be appropriately called hornblendic, augitic, or 

 olivine-bearing camptonites. That the intrusions are of pre-Coal- 

 measure age might justly be inferred by their entire absence from 

 those rocks ; but this was placed beyond doubt by the careful mapping 

 of the Coal Measure base by Mr. Strahan, 4 who found that at Maw- 



1 Quart. Journ. Geol. Sue. rxxv. (1879), 637. 



2 'Geology of Atherstone, etc.,' Mem. Geol. Survey, (1900), p. n. 



3 Proc. Geol. Assoc. TV. (1898), 395. 4 Geol. Mag. (1886), pp. 550, 551. 



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