524 MK. A. HARKER ON ROCKS 



specimens of finer grain the individual minerals are scarcely to be 

 detected by the unaided eye. 



In mineralogical constitution the Cuns Fell diabases present no 

 special ])eculiarities. Apatite prisms occur plentifully, though only 

 locally [92-i]. Crystals and rods of magnetite are always present, 

 but ilmenite is not found. The bulk of the rocks, now considerably 

 decomposed, has been built of felspar and augite, the former in 

 idiomorphic crystals, the latter mostly in ophitic or semi-ophitic 

 plates. The Cuns Fell rock has been termed a gabbro, but its 

 structure is that of a typical diabase, with even an occasional 

 approach to the doleritic type in the development of a few shapeless 

 felspars of later consolidation. The ordinary felspars show albite- 

 and pericline-twinning, and, so far as can be judged from their 

 extinction-angles in rock-slices, may be referred to the border-land 

 between andesine and labradorite. The augite, when unaltered, is 

 colourless in sections, but this mineral is frequently quite destroyed, 

 the common decomposition-products being pale-green delessite, clear 

 quartz, calcite, and opaque dust (kaolin ?). It is, however, fre- 

 quently replaced by hornblende ; sometimes pale greenish-yellow 

 with a fibrous structure and inclusions of secondary magnetite, but 

 more usually clear yellow-brown and pleochroic with a compact 

 structure and good prismatic cleavage. There can be no doubt that 

 this mineral is derived from the augite : the process of conversion, 

 beginning at the margin and along cleavage- cracks, is seen in 

 various stages [925]. The hornblende, as in some other rocks of 

 this kind, gives rather high extinction-angles (cy= about 20°); 

 and it is to be noticed that, in an augite-plate partly transformed 

 into hornblende, the extinctions for the two minerals are, as usual, 

 on the same side of the vertical axis. 



Another common feature of these diabases is the occurrence of a 

 fringe of colourless hornblende growing in crystallographic relation 

 with the crystal-plates, but outside them, and clearly formed at the 

 expense of other minerals, to which it presents a very ragged edge. 

 Such fringes surround not only the uralitic hornblende, but also 

 patches of delessite which are evidently the relics of vanished 

 augite. The order of the several changes indicated is therefore : — 

 (i.) partial or total replacement of augite by hornblende, the uralitic 

 hornblende being perhaps a stage in the conversion to the compact ; 

 (ii.) growth of colourless hornblende-fringes about both hornblende 

 and augite, this proceeding concurrently with alterations in the 

 felspar, &c. ; (iii.) conversion of much of the remaining augite into 

 delessite and other weathering-products. 



The hornblendic rocks examined are from the west and north- 

 west sides of Cuns Fell. On the south-east side, near its junction 

 with the ^Skiddaw Slates, the diabase takes on a finer grain, and, in 

 particular, the felspars occur in long narrow prisms only once or 

 twice twinned on the albite law, without, however, an}^ marked 

 parallelism of disposition [928]. At Dale Eeck the diabase has a 

 very crushed schistose appearance, and a slice shoAVs that it 

 consists mainly of calcite and delessite [926]. Another specimen 



