26 British Association Reports. 



very careful examinations to determine whether metallic iron existed 

 in these rocks (as was the case in the rocks of the Griant's Causeway, 

 in Ireland, according to some analyses), but with a negative result. 



The alteration in external appearance, which had so long led to 

 the supposition that they were so many distinct species confounded 

 under the names of basalt, greenstone, and white feldspathic trap, 

 was due in part to crystallization, in part to the action of water ; and, 

 lastly, in some part also to the fact that ia the smaller veins and 

 sheets, the rock was generally found to contain somewhat less feld- 

 spar and more augite, apparently for the reason that feldspar being 

 the less fusible mineral, had a tendency to cool and become less 

 fluid at a temperature at which the augite was perfectly liquid. 



There appeared therefore to be a greater tendency for the feldspar 

 to retain itseK like a spongy agglomeration of crystals in the main 

 mass of the rock, whilst the more fluid portion travelled furthest 

 into the smaller chinks and cracks of the rocks. The so-called green 

 rock therefore was in general more rich in augite of a green or 

 brown gi'een colour, and was in general more crystalline and coarse- 

 grained, and might easily be mistaken for a greenstone without more 

 minute examination. Commonly also it was to be found more de- 

 composed than the main mass of rock from which it had proceeded, 

 for the reason that having been injected between sedimentary strata, 

 it absorbed water from these beds, or from the springs connected 

 with them. This percentage of water was frequently found to 

 amount to 9 per cent., without much alteration of colour, due to the 

 oxidation of the iron present in the augite. The small white rock 

 or white horse had gone a step further in decomposition — most pro- 

 bably due to the fact that it occurred in general in small veins or 

 strings, and often had absorbed so much water as to be totally 

 altered in appearance, as in the specimen exhibited, which con- 

 tained as much as 20 per cent, of water ; and in fact, at first sight, 

 it looked more like a clay rock, and would not be taken for an 

 igneous rock at all. It must be however remembered that all clay 

 was but decomposed feldspar, and that this feldspar constituted four- 

 fifths of the igneous rocks or dolerites here alluded to ; it was easy, 

 however, to trace the change in situ from the unaltered dolerite to 

 the white rock, or even to the clay which, in its turn, was produced 

 from the disintegration of the white rock. This he had observed in 

 several open sections were the contact of the Eowley Eag and Coal- 

 measures occurred near Dudley, and in the section of the thick coal 

 at the Pensnett Colliery, it might be seen that the decomposition had 

 not yet gone quite so far as to obliterate the peculiar concentric 

 structure characteristic of decomposing igneous rocks of this class. 



When in an unaltered condition the specific gravity of these rocks 

 taken from the centre, or so far from the exterior as to be quite 

 fresh, was wonderfully constant, from whatever part of the Coal-field 

 the specimen might be procured. It might be regarded as 2-84, and 

 this ag-reed well with the usual Titaniferous Dolerites of this class 



