42 DB. STAMP AND ME. WOOLDEIDGE ON THE [vol. lxxix, 



(6) The Intrusion. 



The absence of intrusive types associated with the suite of rocks 

 exposed at Llanwrtyd is rather remarkable. In many other Welsh 

 localities where spilitic rocks occur, ' diabases ' of varying affinities 

 are very abundant. The only rock in the Llanwrtyd district for 

 which an intrusive origin is suggested is that exposed over a limited 

 area near Pen-y-banc Farm. As already stated, no intrusive 

 contacts can be seen, but the lenticular shape of the mass is 

 suggestive, and makes it improbable that the rock is a lava. 



The rock was described by Murchison as a 'greenstone,' and its 

 general appearance is in accord with such a name. Under the 

 microscope, the rock is seen to be composed of a number of 

 chlorite- and bastite-pseudomorphs after olivine, set in a ground- 

 mass of felspar-laths and granular augite. 



The felspar appears to be albite, but it lacks that fresh — almost 

 primary — character of the felspar seen in the spilites, and is clearly 

 the product of a secondary change. 



The augite is pale green to colourless. Some few grains are 

 relatively idiomorphic, but many are shapeless. The patchy 

 appearance of the rock in the hand-specimen (p. 24) seems to be 

 due to the irregular grouping of the augite-granules. 



A few grains of a dark-brown strongly-pleochroic hornblende 

 also occur. It has many of the characters of barkevikite, but its 

 extinction -angles are too high for that mineral. Its optical sign 

 is positive, and it appears to be identical with the mineral described 

 by Dr. Herbert H. Thomas from the marloesites of Skomer Island, 1 

 and by Dr. A. Harker from the mugearites of Skye. 3 It is cer- 

 tainly secondary after augite in some cases. 



The pseudomorphs after olivine present rather unusual characters. 

 In some cases, the original mineral has been replaced by a green 

 chlorite which is nearly isotropic. The characteristic shape of the 

 olivine is often preserved, and the arcuate alteration-cracks (such as 

 are seen in normal serpentinous pseudomorphs) are very apparent 

 (PI. I, fig. 6). Other pseudomorphs present some of the characters 

 of bastite ; they are green and j)leochroic, with a relatively high 

 birefringence and a Avell-marked cleavage. 



Scattered through the ground-mass are large plates of ilmenite, 

 altering to secondary sphene. They are sometimes penetrated by 

 felspar-laths in a subophitic manner. Scapolite also occurs in 

 fair quantity, and presumably contains the lime set free during 

 albitization. 



Since the rock seemed to be related to some of the Skomer- 

 Island types, slides were submitted to Dr. H. H. Thomas, who 

 kindly informed me that the rock compared in some respects with 

 the marloesites of that locality. In his opinion, it was best 



1 Q. J. G. S. vol. lxvii (1911) p. 198. 



2 ' The Tertiary Igneous Eocks of Skye ' Mem. Geol. Surv. 1904, p. 262. 



