TRANSACTIONS OF THE SECTIONS. 55 



to be regretted that in England much looseness prevails in the nomenclature of the 

 plutonic rocks. A microscopic examination of the close-grained and more compact 

 varieties, effects completely that which the naked eye can do in the case of the 

 more crystalline and coarser-grained rocks, and shows them in every case to he 

 composed chiefly of a soda-lime felspar (possibly labradorite) , along with a small 

 amount of augite, and a small amount of titanoferrite or titanite iron, which is 

 never wanting. Other minerals are but seldom present, but traces of zeolites and 

 carbonate of lime may occasionally be found, and possibly a little olivine, though 

 its existence is not as yet satisfactorily proved. The rock is therefore what may 

 be termed a true dolerite, which consequently we must regard as being the sole 

 igneous rock of the district. The presence of titanium is peculiarly constant 

 wherever these rocks are found. However altered and decomposed their appear- 

 ance may be, in the white rock, or in the red clay, on the slopes of the Rowley 

 Hills, which proceeds from the disintegration of the igneous rock itself, there do we 

 find the titanium, which thus furnishes an excellent guide for tracing out the con- 

 nexion with the parent mass. I may mention that I have made very careful exami- 

 nations to determine whether metallic iron existed in these rocks, as in the case of 

 the rocks of the Giant's Causeway in Ireland, according to Professor Andrews, but 

 with a negative result. 



The alteration in the external appearance of these rocks, which has so long led 

 to the supposition that they were so many distinct species, confounded under the 

 names of hasalt, greenstone, and white felspathic trap, is due in part to crys- 

 tallization, in part to the action of water, and lastly, in some part also, to the fact 

 that in the- smaller veins and sheets the rock is generally found to contain some- 

 what less felspar and more augite, apparently for the reason that the felspar, 

 being the less fusible mineral, has a tendency to cool and become less fluid at a 

 temperature at which the augite is perfectly liquid. There appears, therefore, to 

 be a greater tendency for the felspar to retain itself like a spongy agglomeration 

 of crystals in the main mass of the rock, whilst the more fluid portion travels 

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

 therefore, is in general more rich in augite of a green or brown-green colour, and 

 as it is generally much more crystalline and coarse-grained, might easily be mis- 

 taken for a greenstone, without more minute examination. Commonly, also, it is 

 found to be more decomposed than the main mass of rock from which it has pro- 

 ceeded, for the reason that, having been injected between sedimentary strata, it 

 absorbs water from these beds, or from the springs connected with them. This per- 

 centage of water is frequently found to amount to 9 per cent., without much altera- 

 tion of colour, due to the oxidation of the iron present in the augite. The so-called 

 " white rock," or " white horse," has gone a step further in decomposition — most 

 probably due to the fact that it occurs in general in small veins or strings — and 

 often has absorbed so much water as to be totally altered in appearance, as in the 

 specimen here shown, which contains as much as 20 per cent, of water ; and, in 

 fact, at first sight, it looks more like a clay rock, or consolidated clay, and woidd 

 not be taken for an igneous rock at all. If we, however, remember that all clays 

 are nothing' but decomposed felspar, and that this felspar constitutes, we may 

 say, some four-fifths of the igneous rocks or dolerites here alluded to ; it will be 

 found easy to trace the change in situ from the unaltered dolerite to the white rock, 

 or even to the clay, which, in its turn, is produced from the disintegration of the 

 white rock in turn. This I have observed in several open sections, where the 

 contact of the Rowley Rag and coal-measures are observed near Dudley; and in 

 the diagram shown, kindly lent to me by Mr. Beete Jukes for the occasion, and 

 which is from actual survey of Mr. Beckett, and represents the white rock as seen 

 in section in the thick coal at the Pensnett Colliery. Here the decomposition has 

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

 racteristic of decomposing igneous rocks of this class, well seen in the section. 

 When in unaltered condition, the specific gravity of these rocks, when taken from 

 the centre, or so far from the exterior as to he quite fresh, is wonderfully constant. 

 Prom whatever part of the coal-field the specimen may be procured, it may be 

 regarded as 284, and this agrees well with the usual titaniferous dolerites of this 

 class met with in other countries. When altered, as might be naturally expected 



