384 3IK. J. V. ELSDEN ON THE AGE OP [Aug. I904, 



not be lost sight of when utilizing the optical constants for the 

 determination of augites. For, even an incipient weathering may 

 lead to a change in the position of the optical axes. 80 far as I can 

 see in the specimens examined, weathering may produce (1) a 

 lowering of the cy extinction-angle ; (2) a reduction in the value 



of y — a : and (3) an increase in the value of — 5- — • 



Amphiboles. 



In only a single instance have I found a small fragment of an 

 apparently-original hornblende, but secondary amphiboles are 

 represented in a large number of the specimens, more par- 

 ticularly in certain areas where the rocks have been subjected 

 to a particular kind or degree of pressure. Amphibolitization 

 commonly takes the form, in the first instance, of uralite-fringes 

 round the augites, thus bringing these rocks into close agreement 

 with the sills of the eastern part of Caernarvonshire, as described 

 by Mr. Harker; whereas the Lleyn diabases never exhibit this 

 structure. Uralitization is generally associated with pressure- 

 metamorphism, 1 and it is difficult to escape from the conclusion 

 that the same pressure to which the uralitization of the eastern 

 sills was due also operated in the case of the Llanberis dykes. 

 That uralitization is independent of weathering processes pure and 

 simple seems abundantly clear, for the polarization-tints on the urali- 

 tized crystals are often high ; while in the same slide, other crystals, 

 more weathered and showing lower tints, have no trace of uralite- 

 fringes. In partly-weathered crystals, also, uralite is equally well 

 developed on the freshest portion. It may, however, be mentioned 

 that the development of uralite is apparently checked wherever the 

 crystals have secured molecular relief from the effects of pressure, 

 either by the acquisition of strain-slip cleavage, or by mylonitiza- 

 tion. I do not know how far other observers have noticed this 

 feature, which is very well illustrated in these dykes, subjected as 

 they have been to varying kinds and degrees of stress. 



More pronounced alteration of the augite leads to the develop- 

 ment of a pale actinolite and tremolite ; and in some cases fissures 

 and cracks, varying from J to upwards of 2 inches in width, are 

 filled with tremolite or asbestos, which also coats shear-planes and 

 slickensided surfaces. Under the microscope, these features recall the 

 examples of 'gewanderte hornblende' described by E.Cohen, 2 

 Bergt, 3 and Doss. 4 The connection of asbestos with mechanical 

 movement in the containing rock has been already enlarged upon 

 by G. P. Merrill 5 and Van der Bellen, 6 the latter maintaining that a 

 certain plastic elasticity is necessary for its formation. Direct passage 

 of augite into asbestos has been described by J. R. Blum ' ; but in the 



1 J. J. H. Teall ' British Petrography ' 1888, p. 161. 



2 Neues Jahrb. f. Min. vol. i (1883) p. 202. 



3 Tschermak's Min. u. Petr. Mitth. n. s. vol. x (1889) p. 350. 



4 Ibid. vol. xi (1890) p. 46. 



5 Eep. U.S. Nat. Mus. (Smiths. Inst.) 1899, p. 296. 

 ,; Chemiker-Zeitung, vol. xxiv (1900) p. 284. 



r ' Die Pseudomorphosen des Mineral reichs ' 1843, p. 165. 



