184 ME. F. E. COWPEE EEED ON THE GEOLOGY OE [May 1 895, 



Weiselberg pitchstone, 1 and of the crystallites in common window- 

 glass figured by Vogelsang. 2 Liebe has described parallel systems 

 of iron-ore crystallites in an East Thuringian variolite. 3 With 

 crossed nicols felspar-fibres in parallel systems, in the shape of 

 definite crystals and showing twinning, or interlaced and crossing 

 in all directions, become visible ; and a few skeleton- crystals of 

 felspar may also be detected. Rosenbusch figures very similar 

 feathery aggregates and bundles of felspar-crystals in a Caucasian 

 trachyte i ; and the skeleton-crystals remind one of those figured by 

 tbe same author in pi. iii. of his ' Rock-forming Minerals ' (Iddings's 

 translation), and described as occurring in the Weiselberg pitcbstone 

 in the semi-vitreous groundmass. 5 In another slide [101] (PI. YI. 

 fig. 7) the above-described structures are all present, but a further 

 stage in the fusion of the felspatbic fibres into definite crystalline 

 individuals is shown ; and, in addition to long felspathic fibres and 

 numerous skeleton-crystals of felspar, definite porphyritic felspar- 

 crystals with a central semi-opaque axis occur, similar to those 

 described by Cole and Gregory in the variolite of Mont Genevre. 6 



In another rock-specimen, also from Garn Fawr, we see in the 

 section [90] under the microscope that the pale green augite- 

 crystallites lie in places without any order, and are more numerous, 

 more interlaced, more slender, and more ' pinnate ' with side 

 branches. In this section also there are many short, well-formed 

 lath-shaped felspars, with visible twinning and an extinction-angle 

 of about 8°, while skeleton-felspars are very scarce (PL VI. fig. 6). 



Another section from a specimen chipped off the northern craggy 

 spur of Garn Fawr [287] is distinguished by the presence of some 

 large porphyritic lath-shaped felspars occurring in groups and 

 containing inclusions of the groundmass, etc. In all these examples 

 from Garn Fawr the groundmass gives an indefinite reaction with 

 crossed nicols, and appears to have been originally of a glassy nature. 



This taclrylytic 7 variety is not confined to Garn Fawr, but 

 occurs near St. Degan's Church, on the northern coast of Strum ble 

 Head [272]. In this it is worthy of remark that the augite- 

 crystallites and granules are most thickly grouped around the 

 felspar-crystals to which they form a dark border, and in places a 

 local bleaching of the groundmass has taken place by reason of this 

 local aggregation — as in the case of the Eskdale dyke. Near Globo 

 Fawr, Bigney, and Caerlem the tachylytie type is also found [268]. 



In many important features these rocks correspond with the 



1 Eosenbusch, op. cit. 2nd ed. vol. ii. (1887) p. 501. 



2 Vogelsang, ' Die Erystalliten,' Bonn, 1874, pi. viii. 



3 Liebe, ' Schichtenaufbau Ostthiiringens,' Abh. z, geol. Specialk. v. Preussen, 

 vol. v. Heft 4 (1884) p. 110. 



4 ' Mikroskop. Physiogr.' 2nd ed. vol. i. (1885) pi. iii. fig. 4. 



5 In a Pantellerian tacbylyte Forstner bas described similar branching fern- 

 and featber-like felspar-microlites, Min. u. petr. Mitth. vol. xii. (1891) p. 510. 



6 Quart. Journ. Geol. Soc. vol. xlvi. (1890) p. 313. 



7 The term ' tachylyte ' is used here in a broad sense, and corresponds to 

 Zirkel's ' basaltic obsidian,' ' Lehrb. d. Petrogr.' vol. iii. p. 91 (ed. 1894). 



