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ALTERED COALS AND SLATES. 



101 



Specimens illustrative of the effects of heat and intruded Upper 

 melted rocks on slates, coals, sandstones, and conglomerates. ALLERY# 



"Wall-case 45 



1. — Thick coal altered by the intrusion of "white trap 



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age of the Carboniferous rocks. 



rock. 



When this kind of alteration takes place the coal is 

 locally said to be " blacked]' when, by its proximity to 

 an igneous rock, it has become so altered as to lose all its 

 brightness, and nearly if not quite all its inflammability. 

 It is not exactly coke, but is dull and earthy, and, on ex- 

 posure to the atmosphere, is very friable. (" Geology of 

 the South Staffordshire Coal Field/' Jukes, p. 242.) 



2. — Heathen coal of Dudley, in contact with white 

 igneous reck ; rendered anthracitic and seamed by threads 

 of calcareous spar. 



3. — Coal altered by porphyritic greenstone, and enclosing 

 nodules of magneso-calcite, near Bilston furnaces, South 

 Staffordshire. 



The igneous rocks mentioned above are probably of the 



(See "Geology of the 

 South Staffordshire Coal Field," by J. B. Jukes, p. 248.) 

 In the South Staffordshire coal field sheets of greenstone, 

 known in the district as " green rock," have been injected 

 among the Coal measure beds. From these proceed dykes 

 and veins of " white rock," known to be " truly an igneous 

 rock by the way in which it cuts through the coal and 

 other matters, often producing more or less alteration in 

 them at the place of contact." (" Geology of the South 

 Staffordshire Coal Field," p. 241.) 



4. — Specimen exhibiting the action of fire on slate. 



From the premises of Messrs. Scott Russell and Co., burnt 



10th September 1853. Presented by Superintendent Braid- 

 wood, 



At the top of this specimen is a piece of the slate scarcely 

 altered, and showing the original thickness of the slate. 

 Lower down the same fragment swells out and becomes 



