FROM THE CROSS FELL INLIEH. oL") 



that of the JltJioiJii/s(p, in some acid lavas. There are, however, 

 novel peculiarities. The cavity has been at one timo lined with a 

 thin coating of a pale-green chloritoid mineral, which for brevity 

 may be called delessite. It has a fibrous structure, with partial 

 fan-like grouping, roughly per})endicular to the surface on which it 

 was deposited. From this surface it has for the most part broken 

 away, so as to divide the cavity by partition-walls, not however 

 continuous. A second coating of the same material has also to 

 some extent become detached, and with it portions of the andesitic 

 matrix itself. Subsequently crowds of minute but perfectly-formed 

 felspar prisms have been formed, clustering especially, with a ten- 

 dency to perpendicular growth, on the detached fragments of 

 andesite where these were not protected by a coating of delessite. 

 Finally, all the remaining space has been occupied by clear crystal- 

 line quartz. The little felspar-crystals are clear, and invariably 

 have twin-lamellation. The birefringence is very near that of 

 (|uartz, and sections nearly perpendicular to the twin-plane give 

 extinction-angles up to about 18°. These characters do not distin- 

 guish between albite and andesine. The curious feature is the clear 

 evidence that the felspar-crystals were formed within the vesicle 

 subsequently to the deposition of the usual coating of green decom- 

 position-product. 



A few days after the preceding paragraph was written, Mr. W. 

 Maynard Hutchings informed me of his independent discovery of 

 felspar within the vesicles of some Lake District rocks, and the 

 specimen which he kindly lent me showed relations in some respects 

 analogous to those briefly described above. The subject is one 

 which will no doubt repay further investigation, and we may expect 

 that Mr. Hutchings 's work will throw light on this curious mode of 

 occurrence of felspar. 



The rock exposed in Wythwaite Hole seems to be a contempora- 

 neous lava of more crystalline type (dolerite), but is too deeply 

 altered for minute study. Besides evident spherical vesicles, there 

 are seen under the microscope little irregular spaces occupied by 

 (partz-mosaic, but the manner in which the lath-shaped felspars 

 l)roject into these renders it doubtful whether the spaces were 

 originally vacant [1321]. 



A very singular rock occurs on Wythwaite Top. To the eye, it 

 appears a coarse ash or fine breccia. Besides minute glistening 

 felspar-crystals in the general mass, there are little fragments 

 which themselves enclose felspars. In a slice [1322] the fragmental 

 character is scarcely apparent. Idiomorphic felspars are scattered 

 through the rock, showing twin-striation of the ordinary kind, 

 occasionally crossed by pericline-lamellaj. Harely there is a grain 

 of quartz of clastic appearance, or a green pseudomorph which 

 seems to come from a rhombic pyroxene. The general ground of 

 the rock appears in ordinary light partly turbid, j)artly clear, the 

 two occuiTing in intermingled irregular patches. The turbid por- 

 tion presents a finely " felsitic " appearance, but the clear ground 

 consists almost entirely of a mass of perfectly pellucid small crystals 



Q.J.G.S. Is^o. 188. 2o 



