266 ME. K. H. RASTALL ON THE [May I906, 



parts, and the specimen containing a fine-textured groundmass might 

 be described as an augite-porphyrite. But the evidence shows 

 that these peculiar types are hybrid or mixture-rocks; and in this 

 case the mixture seems to have been very imperfect, thus accounting 

 for the patchy structure. The needle-rock occurs near the very 

 irregular boundary between the basic and acid rocks, and is pro- 

 bably due to the action of a second intrusion of more acid character 

 (namely, the granophyre of the largest laccolite), while the earlier 

 basic intrusion was still hot. It is, therefore, probably due to 

 the remixing of two partial magmas previously separated by deep- 

 magmatic differentiation. 



A similar type of ' needle-rock ' also covers a large area on the 

 south-eastern margin of the largest laccolite, on the southern flank 

 of Seatallan. In petrographical character it is very similar to the 

 rock described above, and does not require separate treatment. 



A similar acicular habit of the augite has been noticed by 

 Mr. Alfred Harker x in marginal modifications of the Carroek-Pell 

 Granophyre; and he compares it with the blade-like habit of the 

 biotite on the margins of the Shap Granite and several intrusions 

 of granite-porphyry in other parts of the Lake District. 



(g) The Wastwater District. 



About three-quarters of a mile north-east of Santon Bridge, on 

 the north side of the road to Wastdale Head, are to be seen some 

 exposures of rock of a peculiar character. This marks the south- 

 western limit of the granophyre-intrusion in this district, and is 

 probably another marginal forerunner, like those of Burtness and 

 Bowness. 



In hand-specimens this rock much resembles a rather coarse 

 type of the granophyre, except as to its colour, which is more blue 

 or grey, with a few conspicuous red felspar-crystals. 



Under the microscope [4771] it is seen to consist of a rather 

 coarse-grained aggregate of idiomorphic prisms of plagioclase-felspar, 

 large irregular plates of orthoclase, and some interstitial quartz, 

 with a considerable amount of ferromagnesian mineral. This 

 includes some chlorite and epidote, but the bulk of it is hornblende, 

 which is pale green or brownish and distinctly pleochroic ; much 

 of it shows uralitic character. A few long prismatic crystals now 

 consist of chlorite; and both kinds of ferromagnesian mineral 

 probably represent original augite. Large prisms of apatite and 

 irregular crystals of magnetite are abundant. 



The plagioclase- crystals show strong zonary banding, and the 

 extinction-angle of the middle part is in some as high as 30°, while 

 in the outer zones it is nearly straight. This shows that the felspar 

 ranges from labradorite to oligoclase. 



This rock seems to be another modification of the non-granophyric 

 quartz-plagioclase-augite rock, which is so characteristic of this 

 series ; but it differs from the more ordinary examples in the 



1 Quart. Journ. Geol. Soc. vol. li'(1895) p. 127. 



