213 



The following analyses illustrate the composition of the Rowley dolerite 

 and of the contemporaneous veins : 



I. II. III. 



Si0 2 ... 49-860 ... 48-8 ... 58-3 



Ti0 2 ... 1-330 ... notest. 



A1 2 3 ... 12-750 ... 18-1 ... 17-9 



Fe 2 3 ... 3-360 ... 3-5 ... 2-5 



FeO ... 11-380 ... 7-2 ... 3-0 



MnO ... not est. ... not est. 



CaO ... 8-710 ... 8-4 ... 2-1 



MgO ... 4-395 ... 4-9 ... 1-9 



Na 2 ... 5-250 ... 8-7 ... 5-2 



K 2 ... 0-570 ... 1-9 ... 5-9 



PA .. 0-580 



H 2 ... 2-560 ... 3-6 (Ig.) ... 2-7 (Ig.) 



100-745 100-1 99-5 



Sp. Gr. 2-907 ... 2-79 2-58 



I. Dolerite, Rowley. Analysis by Mr. HENEY. Quoted from the Geological Survey 

 Memoir on the South Staffordshire Coal Field. 1859, p. 117. 



II. Dolerite, Hailstone Hill, Rowley. Mr. WALLER. Midland Naturalist, Vol. VIII., 

 p. 265. The rock contains about 2 p. c. of titanic acid which was not estimated in this analysis. 

 III. Contemporaneous vein (grey) in dolerite from Hailstone Hill. Mr. WALLEE, loc. cit. 



It is interesting to note that the contemporaneous vein contains more 

 silica, less iron, lime and magnesia, more alkali, and relatively more potash 

 than the main mass of the rock (see ante page 42). 



The basic igneous rocks found beneath the surface in the collieries 

 of the South Staffordshire district appear to be in most cases more 

 altered than those just described. When found away from the coal they 

 are usually green in colour owing to the presence of chlorite, chlorophaeite, 

 or other green decomposition products. Zeolites are often abundant. In 

 the neighbourhood of the coal they become light coloured or even white 

 (white trap). Apart from the alteration to which they have been 

 subjected, they bear a close relation to the rocks of the Rowley Hills 

 and the Pouk Hill. Thus a rock from a colliery at Deepmore, two miles 

 N.W. of Walsall, was originally an ophitic olivine-dolerite. In the white 

 trap the alteration has been so complete as to have destroyed altogether 

 the original minerals ; their forms only having been preserved. The 

 white colour is, of course, due to the fact that the carbonaceous matter 

 of the coal has prevented the oxidation of the iron. 



A very interesting type of rock occurs at Tansley Hill, near Dudley. 

 It is a compact rock (basalt) of micro-porphyritic texture. The porphyritic 

 constituents are felspar and occasionally olivine, but they can only be 

 recognized under the microscope. The ground-mass which makes up by 

 far the greater portion of the rock is an aggregate of extremely minute 

 augite crystals magnetite crystals and rods (? ilmenite), and felspar. The 

 felspar mostly gives lath-shaped sections the outlines of which are more or 

 less indistinct (see Fig 2, Plate XXIV.). What is important about this rock is 



