380 ME. J. V. ELSDEN ON THE AGE OE [Aug. 1904, 



plentiful occurrence of compounds of titanium in the Llanberis 

 rocks is of considerable interest. 



Still more important is the evident connection between the altera- 

 tion of ilmenite and the amount of dynamic metamorphism which 

 the rocks have undergone, as Mr. Harker has already noticed in the 

 diabases of Eastern Caernarvonshire. Dr. Teall, also, has found the 

 mineral of great service in tracing the origin of certain schists from 

 sheared diabases. 1 These phenomena are well illustrated in the 

 Llanberis dykes, where every stage in the alteration of ilmenite may 

 be traced as the dykes are followed into regions of increased dynamic 

 influences. An interesting example of this alteration is seen in the 

 production of rutile from ilmenite, as previously described by Prof. 

 Cathrein. 2 This mineral occurs in one of the slides, in the form of 

 abundant hair-like microliths, associated with fragments of still opaque 

 leucoxene. Sphene in distinct granules, as well as the translucent 

 variety usually associated with leucoxene, is abundant. A notice- 

 able feature, however, of many of the yellowish-brown granules is 

 that they do not possess the high double-refraction of sphene, but 

 transmit only a feeble light between crossed nicols. A similar 

 appearance was noticed in the kimberlite of Kentucky by Mr. J. IS. 

 Diller, 3 and by Dr. G. H. Williams in the serpentine of Syracuse, 

 in which cases chemical tests showed these grains to be perowskite. 

 It would not be possible to say definitely that these feebly double- 

 refractive granules in the Llanberis rocks are perowskite, merely 

 on account of their optical anomaly ; but the possibility suggests 

 itself that a part of the rutile liberated from ilmenite has combined 

 with lime to form this mineral. A similar occurrence of this 

 presumed perowskite has been noticed by me in the diabase of 

 the Santon complex in the Isle of Man. It is, of course, not 

 necessary to assume that this mineral has been derived from ilmenite, 

 as its marked association with chlorite-areas might also suggest a 

 derivation from a pre-existing titaniferous pyroxene. 



All the phenomena exhibited by the titanium-compounds in 

 these dykes, both as evidence of a richly titaniferous magma and as 

 proving extensive dynamic metamorphism, are highly characteristic. 

 It is, indeed, possible to trace the kind and degree of pressure- 

 alteration in the successive portions of these dykes by observation 

 of the titanium-minerals alone. 



With regard to other iron-ores, such as magnetite and pyrites, 

 these present the usual characteristics, as described by Mr. Harker 

 in dealing with the Bala diabases, and they do not require further 

 description in this paper. 



Felspars. 



The felspar is always triclinic, and occurs usually in idiomorphic 

 crystals, with well-marked albite-twirming. Pericline-twinning is 



1 'British Petrography' 1888, p. 233. 



- Zeitschr. f. Krystallogr. vol. vi (1882) p. 244. 



3 Bull. U.S. Geol. Surv No. 150 (1898) p. 294. 



