238 J. J. H. TEALL—PETROLOGICAL NOTES ON 
but the same feature was exposed in a small quarry on the east 
side of the dene, a few hundred yards inland. The dyke here was 
about 10 feet wide at the bottom and 5 feet at the top of the quarry, 
the depth exposed being about 20 feet; it terminated upwards in 
an abrupt manner, and the indurated shales which were continuous 
over the top showed a slight upward arching. Macroscopically and 
microscopically the rock bears the closest resemblance to that last 
described. 
Through the kindness of Messrs. Linsby and Ormsby, I was con- 
ducted to two exposures in the Shankhouse pit. The south dyke 
was here seen to be 7 or 8 feet wide, its centre being occupied by 
a rubbly mass containing angular fragments of whin and Coal-mea- 
sure strata; Mr. Paterson, the overman, informed me that this 
was a common feature in the dykes of this district. I presume that 
the centre has in many cases been a plane of weakness, owing to 
cooling from opposite sides, and that subsequent movement has 
taken place along this plane, giving rise to a fault-breccia. The 
northern exposure in this colliery showed two parallel dykes, one 
measuring several feet and the other only 11 inches in thickness. 
These two dykes run together into one within the limits of the same 
royalty. The coal was much charred in the neighbourhood of the 
igneous rock, which was seen in one case to send off narrow irregular 
veins. The narrow dykes and veins were altered to the condition 
of “ white trap ” *. 
Numerous specimens from these dykes as they are exposed on 
the shore near Seaton, in the dene, and in the Shankhouse pit have 
been examined both macroscopically and microscopically. HKvery- 
where they present the same features. The rock is dark, in some 
cases almost black in colour and finely crystalline in texture: Por- 
phyritic elements are very rare, but now and then crystals of felspar, 
similar to those of the Tynemouth rock and doubtless belonging to 
the same species, may be recognized. In some specimens the small 
spherical amygdaloids are notuncommon. Alteration at the surface 
gives rise to the characteristic brown colours; but in the pit these 
are never observed, the large amount of organic matter causing 
the water to act as a reducing, instead of an oxidizing agent. The 
specific gravity of the rock varies from 2°81 to 2-824. 
Under the microscope one recognizes long narrow sections of 
plagioclase, irregular grains and plates of a colourless pyroxene, 
and a certain amount of interstitial matter rendered more or less 
opaque by various devitrification-products, and especially by small 
crystals, rods, and granules of magnetite. In some cases the ulti- 
mate base is seen to be a rich brown isotropic glass. The secondary 
products comprise, as usual, calcite, quartz, pyrites, and a yellowish- 
green substance giving aggregate polarization. The latter product, 
however, is by no means invariably present. A section of the white 
altered rock from the pit shows the felspar fresh, and the pyroxene 
replaced by calcite. 
* Jukes, ‘South Staffordshire Coal Field, 2nd edit. p. 118. 
