10 



THULEAN BASALTS 785 



enstatite-augite. The amounts of magnetite and ilmenite are decidedly 

 high in all the rocks. 



The relations of these basalts to the rhyolitic or toscanitic lavas are 

 little known and can not be discussed here. These more silicic rocks 

 would seem to have been erupted, contemporaneously with basaltic flows, 

 from Tertiary times down almost to the present. With one or two ex 

 ceptions, the analyses that we have of them, such as those of Backstrom 

 are not very satisfactory, but, so far as they go, they indicate a very con- 

 siderable degree of variation, silica ranging from 79 to 63 per cent and 

 the alkalies varying very much in their mutual relations. 



FAROE ISLANDS 20 



These islands are made up very largely of numerous almost horizontal 

 sheets of basalt, which dip slightly through subsidence and which average 

 about 50 feet in thickness. They are accompanied by sills, dikes, and 

 necks. The petrography of these lavas has been little studied, an early 

 paper by Osann 21 and a recent one by Holmes 22 giving almost all the in- 

 formation concerning this that we have. 



It would appear that there are two main types of Faroe basalt: one 

 porphyritic, with phenocrysts of labradorite, and the other dense and 

 aphyric. Holmes describes and gives figures of both types, and there 

 would appear to be little difference between them, except for the feldspar 

 phenocrysts. The only specimen examined by me was collected near the 

 harbor of Thorshavn, on the Island of Stromoe, by Dr. Wright, who very 

 kindly turned it over to me for study. It is a slightly vesicular basalt, 

 showing rather numerous thick, tabular phenocrysts of labradorite, up to 

 about 1 centimeter long, in a densely aphanitic, dark gray groundmass. 



In thin section the Thorshavn specimen shows a porphyritic sub- 

 ophitic texture. The labradorite phenocrysts are rather euhedral, un- 

 twinned for the most part, and without zonal structure. There are no 

 phenocrysts of either pyroxene or olivine. The groundmass is made up 

 in great part of small tables of labradorite, which are multiply twinned 

 and with extinction angles that indicate a composition not quite as calcic 

 as Ab 1 An 2 . Between these, in less amount, are small anhedral grains, 

 rather than true ophitic areas, of a colorless augite, with very few and 

 small grains of olivine, very rare grains of ore, and some small prismoids 

 of apatite. There is considerable interstitial, somewhat cloudy, glass, but 



19 H. Backstrom : Geol. For. Forh., vol. 13, 1891, p. 668. 



20 For the geology of the Faroes, see A. Geikie : Ancient Volcanoes of Great Britain, 

 vol. ii, pp. 191. 192, 256, 322 ; and Cornu and Gorgey : Neues Jahrb., Centralbl., 1908, 

 pp. 675-684 (map). 



21 A. Osann : Neues Jahrb., 1884, vol. i, p. 45. 

 32 A. Holmes : Geol. Mag., 1918, vol. — , p. 199. 



