174 PKOP. T. T. GEOOM ON THE IGNEOFS EOCKS fFeb. IpOI, 



types. Where freshest, the rocks are of a dark bluish colour ; 

 they often weather in a spheroidal manner,^ the weathered portion 

 assuming greenish, brownish, or yellowish colours. The medium- 

 grained kinds, when weathered, often resemble ashy sandstones. 



The specific gravity of the diabases investigated, with the excep- 

 tion of the rotten diabase M 182c (which has a specific gravity of 

 2-51), ranges from 2-61 to 2'75. The higher figure (M 106) is 

 probably nearest to the original, for this rock is one of the freshest, 

 most of the rest being considerably altered. 



The percentage of silica in the specimens analysed by Timins ^ 

 (M 106 & 182) ranges from 43-22 to 4522. The rocks would 

 then appear to be of thoroughly basic, or even of ultrabasic, 

 composition. In all probability (see p. 179) the supposed ash- 

 bands (IX, X, XI, & XII) of Timins {loc. cit.) are weathered 

 diabases: the silica-percentage in these ranges from 35*12 to 

 48-00. 



One of the freshest diabases in the district is M 106 ; Mr. Teall 

 refers to this rock as a typical ophitic diabase.^ M 372 is 

 essentially similar. Under the microscope both these rocks are seen 

 to consist of pale brown allotriomorphic augite, with an extinction- 

 angle of about 41° and prismatic cleavages ; pale green idiomorphic, 

 hypidiomorphic, or allotriomorphic pseudomorphs after an 

 olivine poor in iron, and now consisting chiefly of serpentine 

 (with mesh-structure) and calcite; hexagonal corroded plates of 

 ilmenite, now largely replaced by leucoxene; some apatite; 

 and large laths of tolerably fresh or decomposed plagioclase, 

 probably andesine : these latter are twinned once or many times. 

 In M 106, brushes of long fibres, the microscopic characters of 

 which are those of natrolite, have invaded the felspars; they are 

 grouped in large fan-shaped or sheaf-like bundles. The apatite and 

 iron-ores, the felspar, the olivine, and the augite have crystallized 

 out in the order named. The felspar and olivine were the most 

 abundant ingredients, the ilmenite the least. Octahedra of mag- 

 netite and secondary flakes of biotite are occasionally seen in the 

 serpentine. In a more weathered but otherwise similar rock 

 (M 182 c), the ferromagnesian minerals are altogether replaced by 

 serpentine, calcite, etc. Varieties of fine grain (M 103, 184) show 

 a similar constitution ; in M 184 there is much calcite. 



The marginal facies of a sill observed at Coal Hill Cottage (top 

 of M 182^, PI. VII, fig. 1) shows numerous large isolated lath- 

 shaped and tabular crystals of clear felspar, mostly untwinned, but 

 sometimes twinned several times ; less abundant than these are 

 serpentinous pseudomorphs apparently after olivine, sometimes pene- 

 trated by the felspar-laths, and masses of iron-pyrites. These are 

 embedded in a very fine-grained groundmass of extremely minute 

 laths of felspar, with small patches of serpentine, and many minute 



^ See also Mem. Geol. Surv. Gt. Brit. vol. ii, pt. i (1848) p. 56. 



2 Quart. Journ. Geol. Soc. vol. xxiii (1867) p. 353, Nos. I ( = M 182), and V, 

 VI, &VII(=M106). 



3 ' British Petrography ' 1888, p. 245. 



