from the Lake District. 177 



at or about this horizon are intrusive ; but the rock in question has 

 the characters of a lava, and those with which it is associated are 

 apparently lavas and breccias. Most of these rocks on Sty Head 

 Pass are more or less richly garnetiferous, but this specimen contains 

 no garnet. 



A thin slice (No. 1240 of the Sedgwick Museum collection) shows 

 crystals of cordierite and felspar, with other minerals less abundant, 

 in a fine-textured felspathic ground-mass. The cordierite crystals 

 are about "03 inch long and '02 inch broad, and are invariably 

 complex twins. A cross-section shows a hexagonal outline and, 

 between crossed nicols, the well-known division into six fields, of 

 which each opposite pair have like optical orientation. The mineral 

 is partly fresh, partly converted into a finely divided scaly aggregate 

 of a pale yellowish green colour, which may be identified with the 

 usual 'pinite' alteration-product, probably a mixture of white mica 

 and chlorite. The porphyritic felspars, giving rectangular sections 

 •02 to '05 inch long, usually show twin-lamellae, with the low 

 extinction-angles of oligoclase. There are also untwinned crystals, 

 which seem to be ortJaoclase. A few chloritic and ferruginous 

 patches probably represent decayed augite, and there are occasional 

 small crystals of magnetite and apatite. The ground-mass consists 

 essentially of minute felspar rods with sensibly straight extinction. 

 It thus appears that the rock has trachytic rather than andesitic 

 characters, but it is not fresh enough for a very satisfactory diagnosis. 



It is well known that cordierite occurs in some abundance in 

 certain volcanic rocks, notably in andesites in the Eifel and the Cabo 

 de Gata district. In such cases, even when the mode of occurrence 

 of the mineral proves it to have crystallized from the igneous 

 magma, there is evidence that the magma had been contaminated 

 by dissolving non-igneous material.' A like origin may perhaps be 

 suspected in the case of the cordierite (usually replaced by ' pinite ') 

 in some granites and quartz-porphyries, as in Cornwall, Brittany, 

 and the Black Forest. Nevertheless, there is no obvious reason why 

 cordierite should not occur exceptionally (as corundum undoubtedly 

 does) as a normal constituent of igneous rocks. In the Sty Head 

 lava there is nothing to suggest directly that the magma has enclosed 

 and absorbed foreign material, and the rock occurs in the midst of 

 a thick series of purely volcanic nature. Excepting the Skiddaw 

 Slates at the base, which contain a few unimportant lava-flows, and 

 the Coniston Limestone group at the top, with which the latest lavas 

 are interbedded, the Lake District succession presents an unbroken 

 sequence of volcanic rocks, with no trace of sedimentary material. 

 Without expressing any opinion as to the origin of the cordierite, 

 I leave the question in the hope that it may be elucidated by the 

 discovery of other occurrences. 



1 For a discussion of this question see Teall, Proc. Geol. Assoc, vol. xvi (1899), 

 pp. 61-74. 



DECADE V. — VOL. III. NO. IV. 12 



