400 



Prof. T. G. Bonney. On Specimens of Rock [Jan. 28, 



length;* (3) thin microlifchs of felspar, not seldom about O'Ol inch 

 long, sometimes as much as 0'02 inch. These, from measurement of 

 the extinction angles, may be referred, at any rate in many cases, to 

 labradorite. They may be called skeleton crystals, for not unfre- 

 quently the edges are penetrated by microliths of the other minerals, 

 or they enclose a sort of thin bar composed of them ; in some cases 

 the outline of the latter is a very acute-angled triangle, and a few 

 microliths (usually of the augite) are scattered in advance of the 

 apex along a line in the crystal. These crystals have a tendency to 



" Devitrified " basalt, showing skeletal crystals of felspar in a ground-mass full of 

 crystallites of the same, x 33. 



bifurcate, and to form slightly divergent groups, as shown in the 

 i llu strati on. f The magnetite, also, is often grouped along lines which 

 run at right angles, one to another, as figured by Vogelsang^ and by 

 Zirkel. 



The ground-mass among the skeletal crystals consists of a confused 

 aggregate of the same constituents, but of smaller size, so that they 

 cannot be readily distinguished with powers below a one-eighth 

 objective. It is difficult to be quite certain, but I think that no glass 

 remains. The supposed augite is sometimes slightly vermicular in 

 form. The felspar appears to have consolidated last, and the same, I 

 think, was the case with the larger skeleton crystals, though the mass 

 when they formed must have been sufficiently plastic to allow these 

 to be developed in definite directions for some distance. Mr. Teall's 

 figure || of the Cleveland dyke, from near Preston, presents a general 



* It is difficult to determine their exact nature, but I have little hesitation in 

 identifying them with augite. 



t A somewhat similar, but more curved and variable, grouping in the case of 

 augite is described by E. S. Dana in a compact basalt from Mount Loa, Sandwich 

 Islands. J. D. Dana, ' Characteristics of Volcanoes,' p. 320, &c. 



t In a basalt from near North Berwick ('Die Krystalliten,' Plate XIII, fig. 1). 



' Basaltgesteine,' fig. 54. 



II ' Geol. Soc. Quart. Jl.,' vol. 40, Plate 12, fig. 4. 



