378 MR. J. Y. ELSDEX ON THE AGE OF [Aug. 1904, 



rocks, it is also one of extreme difficulty ; and conclusions, based 

 upon such evidence, can onl}' be drawn with very great caution. 



In considering this portion of the subject, it will not be 

 advisable to recapitulate previous observations any further than 

 will be necessary to compare these rocks with the Bala diabases on 

 the one hand, and the post-Carboniferous dykes on the other. With 

 the former group Mr. Harker has already made us familiar. 1 

 With all the more important features shown by the diabase-sills of 

 the eastern part of Caernarvonshire, these rocks agree down to the 

 smallest detail ; though certain points, notably the frequent occur- 

 rence of secondary sphene and asbestos, but rarely exhibited in 

 Mr. Harker's specimens, become very prominent in some of the 

 Llanberis dykes. The latter rocks are also sharply separated from 

 the post-Carboniferous dykes of Anglesey described by Mr. Harker, 2 

 Mr. Greenly, 3 and Dr. Matley, 4 all of whom agree that the latter are 

 not very basic in character, possess no appreciable titanic acid, 

 have two distinct generations of felspar, and show no conspicuous 

 signs of pressure-metamorphism. The pyroxenes, also, in these 

 younger rocks belong to a later stage of consolidation, and are 

 apparently of a different chemical composition from those about to 

 be described. 



It will be convenient to consider the minerals in the order of 

 their consolidation, and to divide the area into two parts, in 

 accordance with the previously-described differences shown in the 

 field-examination. These will be designated the dynamic or 

 crush-zone of the more yielding sedimentary rocks, and the static 

 or pressure-zone of the Llyn-Padarn ridge. These terms are used 

 for convenience of description only, for it is evident that a crush-zone 

 must also be a pressure-zone of greater intensity. Prof. Bonney 

 lias called attention, in his paper on the crystalline schists of the 

 Binnenthal, 5 to the necessity for differentiating direct pressure from 

 shearing crush ; and he has proposed the term catathlastic for 

 structures produced by the former, in contradistinction to the 

 m} T lonitic structures produced by the latter. The former term, 

 however, does not appear to have been seriously contemplated, and 

 the distinction is not always easy to make, seeing that both structures 

 will be found together. In the present paper, the distinction 

 referred to above is only intended to mark the effects in the 

 rocks described, which are produced by the different kind and 

 degree of pressure in a soft, yielding mass and in the hard 

 resisting buttress against which the forces acted. Perhaps the 

 terms dynamic and static metamorphism, as suggested by 

 Prof. Judd, 6 might be sufficient to describe these two kinds of force 

 exerted upon a rock-mass by great earth-movements. Structurally, 

 all the rocks examined are, or once were, ophitic diabases. They 



1 ' Bala Volcanic Series of Caernarvonshire ' 1889. pp. 75 et seqq. 



2 Geol. Mag. 1887, p. 409 & ibid. 1888, p. 267. 



3 Ibid. 1900, p. 1G0. 



4 Quart. Joum. Geol. Soc. vol. lvi (1900) p. 247. 

 s Ibid, vol. xlix (1893) p. 104. 



6 G-eol. Mag. 1889, p. 243. 



