Tol. 60.] ROCKS OF THE BORROWDALE VOLCANIC SERIES. 7< 



chlorite and epidote. Quartz occurs in irregular blebs, frequently 

 bordered by a ring of minute augite-crystals ; pyrites is often asso- 

 ciated with the quartz, and encloses the augite-crystals. A striking 

 feature of the rocks of this dyke is the abundance of oval masses of 

 infiltrated quartz with a kernel of calcite. On account of this 

 occurrence,, a chemical comparison of the different types would be 

 futile. In the coarser rocks, oligoclase occurs in long laths in the 

 groundmass : the augite is altered to epidote and chlorite. 



A large intrusion seen near Buscoe Sike, south of Bow Fell, 

 shows the same gradation of rocks as the dyke, and no doubt the 

 two are connected. One of the most basic types (85) is a true 

 porphyrite with phenocrysts of andesine, or an acid labradorite 

 showing beautiful zonary banding, Carlsbad and albite-twinning. 

 Garnet occurs, surrounded by a ring of felspar-crystals of the same 

 species : iron-ore is separating out from the garnet, and apatite is 

 also produced. The augite is represented by a greenish product, 

 plentifully charged with grains of iron-ore. The groundmass is 

 composed chiefly of plagioclase-laths with iron-ore grains. 



Closely connected with the Bow-Fell dyke just described, is a 

 mass of breccia occupying a considerable area west and north-west 

 of Ore Gap. It contains fragments from 1 to 2 feet in diameter. 

 When examined carefully, these fragments are found to be exactly 

 similar to the varieties of the dyke-rock first mentioned. In addition 

 to these, fragments of ' streaky ' lavas and ashes are also included ; 

 and it is from this breccia that the best example of a 'streaky' 

 lava was obtained. Another curious feature of this breccia is the 

 abundance of quartz-blebs. Under the microscope (3768) lapilli of 

 an andesitic rock are extremely abundant, the lath-shaped felspars 

 of which occasionally show a kind of flow-structure. Fragments of 

 the above mentioned dyke-rocks can be recognized, containing the 

 ferromagnesian mineral (altered to epidote) and brownish pleochroie 

 chlorite. Bounded crystals of quartz with their corrosion-borders 

 preserved are abundant, and these are probably derived from 

 the north-north-westerly and south-south-easterly dyke. Garnet- 

 fragments, and portions of the same mineral with well-developed 

 faces, are not uncommon. 



Since the dyke from which the breccia-fragments are presumablv 

 derived is later than the surrounding rock, a question arises as to 

 the age of the breccia. So far as could be gathered from a hurried 

 traverse, the breccia-mass has its greatest length in a northerlv- 

 and-southerly direction, parallel to the iron-lode and fault of Ore 

 Gap : consequently, it would be reasonable to suppose that the breccia 

 originated through crushing. There are, however, none of the 

 usual indications of crush-brecciation, and the mass seems to differ 

 in no way from an ordinary explosion-breccia. Crush-brecciation 

 would, moreover, hardly account for the fragments of ; streaky' lava. 



A detailed mapping of the breccia, and a further study of 

 the behaviour of the few dykes that approach its margin, are 

 necessarv before any theory of a volcanic vent can be brought 



