376 DR. C. CALLAAYAY OX THE METAMORPHISM OF [Aug. 1 898, 



It will have been noticed that the signs of compression and con- 

 tortion become more marked as we descend in the series, and the 

 suggestion arises that a causal connexion exists between the 

 increased metamorphism and the increased pressure. This point 

 will be discussed atter the microscopic evidence has been presented. 



III. Microscopic Eyidince of the Metajsiorphism. 



Some of the rocks of this area were microscopically described by 

 Prof. Bonney in an Appendix ^ to one of my papers on tbe Archaean 

 Geology of Anglesey. These specimens were placed by him in two 

 distinct groups: (1)' Slaty and other not highly altered Eocks,' 

 (2) • Chlorite Schists.' As the object of this paper is to prove that 

 the latter are a more highly altered phase of the former, it will be 

 well to reproduce Prof. Bonney's very succinct descriptions of a few 

 of the varieties. 



The folio wirg is fairly typical of the former group : — '■ 47 (Llan- 

 fechellj. A coarser fragmental rock with a rather streaky struc- 

 ture, not unlike some of those which occur in the Borrowdale Series, 

 containing numerous microliths of the viridite group, some being 

 certainly chlorite. The aspect of the rock suggests that it has 

 undergone considerable pressure. Many of the embedded fragments 

 are from about 0"03 to 0'06 inch in longer diameter. Among them 

 quartz, felspar, altered biotite (?), and a chloritic quartz-schist may 

 be recognized, detrital materials almost certainly derived from the 

 older gneissic and schist-rocks of this region of Xorth "Wales. 

 Other fragments of a less certain character are present, with grains 

 of decomposed ilmenite or magnetite and of epidote, which perhaps 

 has replaced some other mineral.* 



This specimen belongs to the middle group, and is rather more 

 crystalline than the gritty seams in the Khosbeirio Shales. 



The next two slides (45. 46) belong to the second division, and 

 are placed by Prof. Bonney under the head of ' a grou}) of highly 

 altered, distinctly-foliated rocks, consisting mainly of rather minutely- 

 crystalline chlorite and quartz.' Of Is'o. 45 he writes : — ' chloritic 

 constituent rather minute,' and of Xo. 46, ' highly altered and 

 markedly foliated.' 



A series of slides from Bhosbeirio, Llanfechell, Pant-y-Glo, and 

 intermediate localities, links together the fragmental rocks with 

 the true schists. The gradation is seen in both the fragments and 

 the matrix. 



The fragments in the grits are mainly of quartz, but in some 

 of the slides felspar, both twinned and untwinned, is fairly abundant. 

 The bits of schist referred to by Prof. Bonney are rare, but flakes 

 of a clear minutely granular rock are scattered here and there. The 

 quartz is usually angular, and the felspar-crystals are rarely, if 

 ever, unbroken. The shaly seams in the grits, as well as the Rhos- 

 beirio Shales, often contain fragments of quartz and felspar, but 

 they are usually very minute, 



^ Quarr. Journ. Geol. Soc. vol. sxxvii (1881) pp. 232-234. 



