Yol. 57.] SEAiT OF COAL I>ITO A SEA].I OP DOLOMITE. 301 



Another variety of the rock (E 32S0 : see PI. XI I, fig. 2) is built 

 up of small masses or short irregular layers of crystalline matter, 

 separated by very irregular patches of fine mud, in which miuute 

 grains of quartz and mica are recognizable. The layers have been 

 irregularly deposited, some upon the surfa,ce of an inclusion of mud, 

 others upon previously formed crystalline material, but the principal 

 axes of the crystals are always at right angles to the surface of 

 deposition. This rock diifers from the pisolite merely in the fact 

 that the dolomite has tended to coat irregular surfaces, whereas 

 in the pisolite it has crystallized round a number of independent 

 centres. 



The analysis of this rock is tabulated by Dr. Pollard on p. 303. 

 The ignited residue amounts to 23* 68 per cent., and shews the 

 presence of alkalies, derived no doubt from the mica ; the large 

 residue is probably due to the inclusions of mud referred to above. 



Another specimen (E 3344 & 3345 : see PI. XII, fig. 5) consists 

 partly of woody tissue filled with dolomite. It includes also thin 

 bands of coal, which however have been broken up and recemented 

 by dolomite. The perfect angularity of the fragments shews that 

 the coal had hardened before it was broken, and it is possible that 

 the brecciation was subsequent, like some cracks which traverse the 

 original structures in many of the slides. It should be remembered, 

 however, that fragments and pebbles of coal are not uncommon 

 in Coal-Measure conglomerates, shewing that hardening followed 

 rapidly upon deposition. The fragments of woody tissue, after 

 removal of the dolomite by acid, yield a copious residue of minute 

 carbonized vegetable fibres. 



Dr. Pollard's analysis of this specimen, tabulated on p. 303, 

 proves that it is approximately pure dolomite, with the addition of 

 17'80 per cent, of coaly matter and 5*38 per cent, of ferrous oxide. 

 The ignited residue, on the other hand, is extremely small ; the coaly 

 matter may be attributed to the vegetable tissue, and the specimen 

 may be regarded as being made up of fragments of wood impregnated 

 with, and cemented together by, dolomite. 



Other specimens shew small masses and films of coaly matter, 

 not displaying any trace of organic arrangement, but tending to 

 split up and ramify irregularly through a dolomitic matrix (E 3279 

 & E 3281 : see PI. XII, figs. 3 & 4). Crystallization has evidently 

 taken place in the presence of coaly matter, but the dolomite in 

 crystallizing out has rejected the mud and split it up into an 

 infinite variety of feathery forms. 



The phenomena are not those of a ' wash-out.' In such a case 

 the coal, together with some of the associated strata, has been 

 washed away immediately, or soon after deposition, and its place 

 taken by sand, gravel, or mud, the erosion having obviously been 

 due to running water. In the Wirral dolomite not only is there no 

 sign of erosion, but there is proof that it was formed in almost 



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