200 



ME. H. H. THOMAS ON THE 



[May 191 1, 



clear albite. On Grassholm Island in the South Gut a dark-green 

 ferruginous epidote occupies the cavities in one of these lavas. 

 Mineralogically, these rocks are related to the skomerites, from 

 which they differ chiefly in the large proportion of olivine and in 

 the presence of the brown hornblende. They are also related to 

 the olivine-soda trachytes, which differ from them in containing 

 little or no augite and possessing a trachytic ground-mass. 



Compared with rocks of other districts, they present several 

 points of resemblance to some of the less acid rocks of Pautelleria, 

 described by Foerstner, 1 which stretch from the Montagna Grande to 



VII. 



VIII. 



52-37 



49-80 



116 



1-73 



18-05 



16-24 



3-40 



3-85 



4-27 



5-31 



0-28 



027 



not fd. 



0-02 



002 



02 



6-18 



4-01 



5-53 



4-52 



1-97 



2-51 



4-3C. 



4-88 



trace 



trace 



032 



1-08 



1-83 



243 



018 



0-18 



017 



3-22 



100-09 



10007 



E. G. Kadley, 



E. G. Radley. 



Si0 o 



TiO", 



A1,,0, 



Fe 2 3 



Feb 



MnO 



(CoNi)O.... 



BaO 



CaO 



MgO 



K,G 



Na 2 



Li 



H 2 Oatl05°C 



H.;o above 105° C. ... 



P.,0- 



co 2 



Totals 



Anal 



VII = Marloesite, lava-flow ; south side of Grassholm Island. [Anal. No. 330.- 

 Slide E 6995.] 

 VIII=Marloesite, lava-flow ; foreshore, east side of Martin's Haven, Marloes 

 (Pembrokeshire).' [Anal. No. 343. Slide E 7753.] 



Monte Gibele. One of these rocks has been figured for comparison 

 with the marloesites (fig. 10, C, p. 198). It shows large badly- 

 formed olivines (between true olivine and fayalite) in association 

 with anorthoclase. The other porphyritic constituent is a pale 

 monoclinic pyroxene, and the ground-mass contains many sub- 

 ophitic grains of augite, as also a good deal of cossyrite. The 

 brown hornblende of the marloesites, if original, might be taken 

 as representative of the cossyrite of the Pantellerian rocks. 



Chemically (see Analyses VII & VIII, above), the marloesites 

 appear to be derived from a magma which differed but little from 

 that of the mugearites (see Analysis XII, p. 202), except in the 

 smaller percentage of iron-oxides and the greater percentage of 

 magnesia : pointing to a smaller proportion of iron-ores and an 



1 ' Geologia dell' Isola di Pautelleria ' Boll. R. Comit. Geol. Ital. vol. xii 

 (1881) p. 523. 



