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The rock of the Rowley Hills presents some interesting points of 

 difference when compared with that of Pouk Hill. It is, as a rule, some- 

 what finer in grain, but contains local patches which are extremely coarse, 

 and in which the augite exhibits a tendency to run out into large, bladed 

 crystals. It contains also red and grey contemporaneous veins which differ 

 in composition from the main mass of the rock. In this section the 

 constituents of the Rowley mass are seen to be the same as those of Pouk 

 Hill ; but the augite and the felspar do not exhibit the same relations. 

 The augite in the Rowley rock frequently shows more or less definite form 

 and crystals of this mineral are sometimes seen to lie in the felspar. At 

 the same time there is evidence that the whole of the augite did not 

 separate before the felspar, for a lath-shaped section of the latter mineral 

 may sometimes be seen to penetrate a grain or imperfect crystal of 

 augite. In some of the coarsely crystalline patches Mr. ALLPORT 

 observed a remarkable intergrowth of felspar and augite (see Fig. 2 

 Plate XXIII.). The two minerals form globular aggregations and, though 

 intimately intermingled, each possesses uniform optic properties over large 

 areas. This is an illustration of the ocellar structure of FISCHER, the 

 centric structure of BECKE, and the micro-pegmatitic structure of 

 French authors. 



It is interesting to note that in South Staffordshire we have illustrations 

 (1) of dolerites in which the felspar has preceded the augite, (2) of dolerites 

 in which the two minerals have crystallized simultaneously, and (3) of 

 dolerites in which the felspar has succeeded the augite. llmenite in good- 

 sized hexagonal tables is found in the Rowley rock, and is sometimes 

 collected in considerable abundance in special localities. The olivine of the 

 Rowley rock is often distinctly coloured in thin section and evidently 

 belongs to a variety extremely rich in iron. It passes over into deep green 

 alteration products. The observations of Mr. ALLPORT and Mr. WALLER 

 show that the mass is by no means uniform in structure or composition. 

 In some varieties olivine appears to be absent, in others it is very abundant. 

 The rock is usually granular ; but micro-porphyritic varieties are sometimes 

 found. In these the ground-mass is formed of very small crystals of 

 felspar and augite with a considerable amount of magnetite. 



The grey contemporaneous veins consist of perfectly fresh felspar, 

 green augite and interstitial matter which is partly at any rate in the 

 condition of glass. The felspars are seen under the microscope to be either 

 simple individuals or binary twins. The striation so common in plagioclase 

 is absent. Many of the binary twins extinguish simultaneously when the 

 trace of the face of composition lies parallel with one of the vibration 

 planes. Orthoclase must therefore be present. The interstitial matter 

 sometimes contains slender felspar-microlites which are often beautifully 

 curved. The green colour of the augite is interesting as it is in marked 

 contrast with the brown colour of the normal augite of the rock and 

 similar to that of certain acid rocks (Mull granophyres), and of certain 

 rocks rich in alkalies, 



