211 



trusive, and therefore some difficulty is experienced in determining their 

 age. The entire absence of similar rocks in the strata overlying the Coal- 

 Measures (New Red Sandstone), and the fact that they have been affected 

 by the same dislocations as the Coal Measures, led Professor JUKES to refer 

 them either to the later portion of the Carboniferous period or to the im- 

 mediately succeeding period. In any case they seem to have been formed 

 before the New Red Sandstone period. The basic igneous rocks associated 

 with the Coal-Measures of Leicestershire and Shropshire doubtless belong 

 to the same period. 



The petrographical characters of the rocks have been described by 

 Mr. ALLPORT. (1; Mr. WALLER has added some very interesting facts with 

 regard to the contemporaneous veins in the Rowley Rag mass. (2) To a 

 certain extent these rocks must be regarded as classical in the history of 

 petrography as they furnished to Mr. ALLPORT absolute evidence that rocks 

 identical in structure and composition with Tertiary dolerites have been pro- 

 duced in very early geological times. 



The normal rock of Pouk Hill is a typical ophitic olivine dolerite. 

 " A specimen from the centre of a column near the middle of the large 

 quarry exhibits in a thin slice, the whole of the constituents in a remarkably 

 fresh condition. The plagioclase occurs in the usual long prisms and is 

 distinctly striated. The augite is in grains of irregular shape, and is 

 frequently penetrated by the felspar ; its colour varies from yellowish to 

 purplish brown ; and some of it is slightly dichroic. Olivine is extremely 

 abundant, and is often nearly or even quite unaltered; other crystals, how- 

 ever, are partially converted into a green substance round the edges and 

 along the sides of fissures ; a little of the green substance has also found 

 its way here and there among the felspar prisms. The never-failing mag- 

 netite occurs in moderate quantity ; and there are a few needles of apatite, 

 together with an occasional small patch of amorphous glass. The rock is 

 quite undistinguishable from a Tertiary dolerite, either in mineralogical 

 composition or state of preservation." < 3) The microscopic structure of this 

 rock is essentially similar to that of the rock from Tobermory represented 

 in Fig. 1, Plate X. The principal difference lies in the colour of the augite, 

 which is much more pronounced in the case of the Pouk Hill rock. The 

 sheet of dolerite overlying the Coal-Measures and underlying the Trias met 

 with in the course of colliery workings at Whitwick, in Leicestershire, is 

 similar in structure and composition to the rock from Pouk Hill above 

 described. 



In addition to the ophitic olivine-dolerite there occurs also at Pouk 

 Hill, near the bottom of the cutting, a rock of micro-porphyritic structure. 

 Comparatively large crystals of felspar and olivine (pseudomorphs) lie 

 embedded in a compact matrix " consisting of minute grains or crystals of 

 felspar, augite and magnetite." 



(1) G. M., 1870, and Q. J. G. S., Vol. XXX. (1874), p. 529. 



(2) Midland Naturalist (1885), Vol. VIII., p. 21. 



(3) Mr. ALLPORT, loc. cit. 



