210 



part changed to serpentine, lath-shaped plagioclase, augite which may 

 occur either in the form of grains or ophitic plates (see Figs. 1 and 2 

 Plate IX.), and magnetite or ilmenite. The rock is therefore an olivine- 

 dolerite which shows in certain parts the ophitic type of structure, in 

 others the " type with granular augites." Indications of fluxion structure 

 may be seen in the latter. The relations between these two types of 

 structure have been already considered in describing the Tertiary 

 dolerites of the west of Scotland. It is interesting to note that the 

 Tides well Carboniferous rock is exactly similar to the Tertiary olivine- 

 dolerites of Tobermory, Portree, and Portrush, both as regards structure 

 and composition. The specific gravity of the least altered specimens 

 of the Tideswell Dale rock is 2'91. 



In Cavedale, near Castleton, a single column of a black basaltic 

 rock is exposed. It is finely crystalline and possesses a specific 

 gravity of 276. Under the microscope it is seen to be composed of 

 lath-shaped plagioclase, colourless augite-grains often more or less altered, 

 rods of magnetite and a considerable quantity of interstitial matter 

 crowded with minute grains and rods of magnetite and often changed to 

 a green alteration product. Felspar is more abundant than augite. 

 Notwithstanding the thoroughly basaltic aspect of this rock, it is evidently 

 not a true basic rock. It belongs to the border line between the 

 rocks of basic and those of intermediate composition, and is thus 

 allied to the andesitic dolerites (augite-andesites). (1) 



Mr. ALLPORT has described the rocks from the neighbourhood of 

 Matlock. 12 ' They form two bands in the Carboniferous limestone and 

 may be examined on the railway near the entrance to the tunnel through 

 the High Tor, as well as on the opposite side of the valley at the Heights of 

 Abraham. A specimen from the upper band at the latter locality is a 

 typical olivine-dolerite of the Tideswell Dale type. A specimen from the 

 lower band contains a considerable amount ol interstitial matter rendered 

 opaque, except in the very thinnest sections, by minute grains of 

 magnetite. These rocks contain amygdaloidal or pseudo-amygdaloidal 

 cavities filled with calcite and chlorite. 



Various exposures of basic igneous rock occur in association with the 

 Carboniferous strata of the Midland Counties, in Leicestershire, Stafford- 

 shire, and Shropshire, The largest of these is that of the Rowley Hills, near 

 Birmingham. These hills have a capping of dolerite resting on Coal- 

 Measures, and extending over " an irregular space two miles long by more 

 than a mile in width, at one part." (3) Other exposures in the South Staf- 

 fordshire Coal Field occur at Barrow Hill, two miles west of Dudley, and at 

 Pouk Hill, near Walsall. Intrusive sheets and veins of igneous rock are 

 also constantly met with in the course of the colliery workings. Where 

 they come in contact with the coal they have been frequently converted 

 into " white trap." In most cases, if not in all, the igneous rocks are in- 



(1) Figs. 1 and '2, Plate XIV., may be taken to represent the Cave Dale rock. 



(->) Q. J. G. S., Vol. XXX., 1874, p. 551. 



(3) On the South Staffordshire Coal Field. Memoirs of the Geol. Survey, 1857. 



