Vol. 60.] THE LLYH-PADARH DYKES. 385 



present case it appears to be derived from uralite or trernolite. as 

 stated by Dr. Hintze. 1 The exact nature of the change is at pre- 

 sent only a matter of supposition. It is not a paramorphic change, 

 since some hydration takes place: intact, all the phenomena 

 connected with amphibolitization in general point to 

 the effects of dynamic metamorphism. 



In connection with this portion of the argument, it may be well 

 to recall the observation of Prof. Grenville Cole & the late A. V. 

 Jennings on the northern lace of Mynydd-y-Gader, 2 where the 

 intrusive diabase also shows a great deal of actinolite and trernolite, 

 with greenish asbestos in the clefts, pointing in their opinion to a 

 magma rich in alumina and lime, rather than to magnesia and iron. 



The occurrence of asbestos in such quantity as is found in 

 some parts of the Llyn-Padarn dykes, notably along the Afon Goch 

 and on the western shore of the lake, near the mouth of the tunne], 

 seems to me to have an important bearing upon the separation of 

 these intrusions from any eruptions of post-Carboniferous age in 

 this part of the country. 



Biotite. 



This mineral is very sparingly represented. Several of the less- 

 altered specimens contain a few shreds partly altered to chlorite. 

 In the crushed rocks no trace of it appears to be left. This fully 

 agrees with the character of the Bala sills, and it seems unnecessary 

 to dwell further upon this point. 



Chlorite. 



The chlorite-areas seen in these rocks have a well-marked 

 relation to the amount of shearing which they have undergone, 

 and are in inverse proportion to the remaining augite. More 

 than one variety of the chlorite-family appears to be present, and 

 they present the following characters : — (1) green, radial, fibrous 

 scales, with parallel extinction and marked pleochroism, possibly 

 representing pennine or ripidolite ; (2) granular aggregates ; 

 and (3) isotropic, structureless patches : these may be delessite and 

 chlorophaeite respectively. The first variety would, therefore, 

 belong to the true chlorites, and the two latter to the saponites 

 of Dr. Heddle's classification.'' It is possible, however, that the 

 saponites are only more hydrated forms, and may be derived from 

 the chlorites by simple weathering processes. In the more highly- 

 sheared varieties the chlorites are drawn out into distinct lenticles, 

 showing a passage into flaser-diabase (the early stage of a chlorite- 

 schist) as has been alreadj" pointed out by previous observers, as 

 the result of the metamorphism of diabase by earth-stresses. 4 



1 Handbuch der Mineralogie, vol. ii (1897) p. 1195. 

 - Quart. Journ. Geol. Soc. vol. xlv (1889) p. 432. 



3 Trans. Eov. Soc. Edin. vol. xxix (1880) p. 55. 



4 See J. J. H. Teall, Quart. Journ. Geol. Soc. vol. xli (1885) p. 133; T. G. 

 Bonney, ibid. vol. xlix (1893) p. 94: T. G. Bonnev & C. A. McMahon. ibid. 

 vol. xlvii (1891) p. 489 ; S. Hyland, Geol. Mag. 1890, p. 205 ; and F. Zirkel, 

 • Lehrbuch der Petrographie " 2nd ed. vol. ii (1893) p. 730. 



U. J. G. S. No. 239. :> o 



