480 PETROLOGICAL ABSTRACTS AND REVIEWS 
Tuomas, Herspert Henry. ‘‘The Skomer Volcanic Series (Pem- 
brokeshire),” Quart. Jour. Geol. Soc., LX VII (1911), 175-212. 
Figs. 13; map 1, and analyses. 
The extreme west coast of Pembrokeshire, England, with the immedi- 
ately outlying islands to the west, of which Skomer Island is the largest, 
consist for the greater part of a succession of lava flows. The rocks 
furnish some unusual mineral associations; and two new rock types are 
developed and described. A descriptive bibliography relating to the 
geology of Skomer Island is given. 
Stratigraphic evidence shows the volcanic rocks to be pre-Upper 
Llandovery, probably lower Arenig in age. The rocks are mainly a 
succession of thin subaerial lava flows of considerable lateral extent 
with which are associated a few doleritic sills representing a later intrusive 
stage. Interbedded with the volcanics are thin beds of conglomerates, 
quartzites, and red clays. 
The petrography of eight distinct types of igneous rocks with several 
variants is described in detail, and their geographical distribution given. 
The rocks include soda-rhyolites and felsites, soda-trachytes, kerato- 
phyres, mugearites, olivine basalts, and olivine dolerites; and the two 
new types skomerite and marloesite. 
These two new types are porphyritic rocks of rather basic composi- 
tion, characterized by the association of a high proportion of albite 
with olivine and augite. The name skomerite is applied to a rock con- 
sisting of porphyritic laths of somewhat altered albite-oligoclase, 
subidiomorphic augite, and small idiomorphic crystals of olivine, in a 
groundmass of unoriented albite-oligoclase laths. Marloesite is de- 
scribed as containing glomeroporphyritic crystals of albite-oligoclase and 
altered olivine in a fine-grained groundmass of subidiomorphic augite, 
albite microlites, soda-bearing hornblende, and accessory iron minerals. 
The more acid rocks of this series show interesting mineralogical 
variations from the generally recognized mineral associations. As the 
most notable peculiarity, these rocks contain soda-rich feldspar pheno- 
crysts in intimate association with porphyritic crystals of olivine, hyper- 
sthene, and augite. The author recognizes the possibility of an original, 
more basic character of the rocks and a subsequent increase in soda- 
rich feldspar by albitization, but concludes, on good evidence, that 
the albite for the dominate part is original. 
The rocks show an extrusive sequence from acid to basic and basic 
to acid, with a resulting frequent repetition of the same types of rocks. 
A. W. STICKNEY 
