SOME ANCIENT AND MODERN VOLCANIC ROCKS. 407 



If the figures 5 and 6 be compared, it will be seen that the struc- 

 ture of the base is very similar in the two cases. In the Torre-del- 

 Greco lava (fig. 5) some of the more transparent green parts of the 

 base are faintly dichroic, as in the old Cumberland trap. In the 

 former, however, leuoite occurs filling up the interstices, which 

 appear dark under crossed prisms; while in the latter there is no 

 absolute darkness when the Nicols are crossed, since the felspar, 

 which seems here to take the place of the leucite, remains more or 

 less transparent, and the dirty green matter, where thickest, is only 

 partially dark. 



2. Brown Knotts, West of Bleaberry Fell, near Keswick (fig. 7). 

 — This is but one of a series of lava-beds cropping out along the hill- 

 side between the eastern edge of Derwentwater and the summit of 

 Bleaberry Pell, and which will be found all fully described in the 

 Survey Memoir. It is the basement -bed of a succession of flows 

 forming Brown Knotts. The flow itself probably averages not more 

 than 15 feet in thickness, rests upon partially altered and false- 

 bedded ash, and presents a very slaggy-looking and vesicular top. 

 The heart of the rock is very compact, fine-grained, of a grey-blue 

 colour, showing a few very small, soft, black spots, and some very 

 minute felspar crystals, with an occasional brick-red, earthy spot 

 here and there. It breaks up very much along numerous weathered 

 joint-lines, and altogether presents an excellent example of an ancient 

 lava. 



Examined microscopically, the general structure is that shown in 

 fig. 7. The base consists of numberless acicular felspar prisms and 

 magnetite crystals, between which is an abundant development of 

 yellowish-green and brownish mineral, scarcely if at all dichroic. 

 "Within the base are scattered small crystals of plagioclase and ortho- 

 clase felspar, singly and in groups ; one of the latter is seen in the 

 figure. Yery little unaltered augite is apparent. Every here and 

 there are vesicles filled with calcite and green earth, presenting the 

 appearance, under crossed Nicols, of dark irregularly shaped spots, 

 surrounded by a coloured edging. Much of the diffused mineral 

 of the base may not unlikely be pseudomorphic matter after augite. 

 There are a few clear green pseudomorphs (a part of one is seen 

 in the figure) which show a faint dichroism and are very likely 

 chlorite replacing other minerals. Zirkcl remarks of chlorite*, " on 

 account of its optically uniaxal character, a horizontally cleaved 

 chlorite plate can develop no dichroism; but chlorite plates cut 

 obliquely or even at right angles to the basal plane, when tested with 

 a single Nicol, exhibit somewhat feeble dichroism, giving rise to only 

 a little change from clear to dark tints of green." 



When the Nicols are crossed, the base presents the same general 

 appearance as in the last example, only that the diffused mineral 

 seems on the whole somewhat darker. 



The following is an analysis of this rock ; and beside it I place 

 that of another lava-bed from the same hill, presenting lithologically 



* Mikroskopische Bescbaffenheit, p. 190. 



