,8,,. SOME NEW MEMOIRS. 229 



the constituents. If any doubt still remained as to the possibility of 

 the most rigid rocks becoming plastic under earth-stresses, it would 

 be removed by these remarkable experiments. 



In the concluding part of the paper, the author discusses the 

 bearing of his experiments on the ejection of masses of rock, molten 

 or otherwise, from the interior of the earth through the channels of 

 communication opened by explosive action. He points out that 

 volcanoes are distributed so that in one and the same district there 

 is a general uniformity in the heights of the principal vents. The 

 elevation characteristic of any particular group is, according to him, 

 a measure of the intensity of the gaseous pressure capable of being 

 developed in the subterranean regions, from which the volcanoes of 

 that group derive their energy. 



Contributions to the Knowledge of the Liparites of Iceland. By Helge 

 Backstrom. Geol. Form, i Stockholm Forhandl., vol. xiii., pp. 637-682 (1891). 



This is a petrographical description of the acid igneous rocks of 

 Iceland. It is of interest to British geologists because it confirms 

 and extends the conclusion, arrived at by previous observers, that 

 there is a close similarity between the rocks of Iceland and those of 

 the North-east of Ireland and West of Scotland. 



Acid igneous rocks are now known from many parts of Iceland. 

 They occur as lava streams of very recent date, and also as dykes, 

 stocks, and sills, intrusive in the basalts. The principal types are 

 classed as liparite, obsidian, pumice, and granophyre. The most 

 striking chemical fact is the very general predominance of soda over 

 potash. Mineralogically, the liparites are composed of felspar, 

 pyroxene, iron-ore, zircon, and glass. Quartz, tridymite, apatite, and 

 olivine have also been observed ; and, in a few cases, hornblende, 

 biotite, hypersthene, and titanite. The porphyritic constituents are 

 principally striated felspar and pyroxene. The silica percentage 

 ranges from 63 to 79. In the frequent occurrence of striated felspar 

 and pyroxene among the porphyritic constituents, and in the 

 relative proportions of the alkalies, there is a resemblance between 

 the Icelandic liparites and the pitchstones and felsites of Eigg and 

 Arran. 



The granophyres also furnish a most interesting point of 

 resemblance between the British and Icelandic areas. These occur 

 as intrusive rocks in the basalts of both areas. They were discovered 

 in situ by Thoroddsen on the north coast of the Snaffels peninsula in 

 West Iceland, and by Helland in three localities in South-east 

 Iceland. In the former locality, plagioclase is the oldest constituent 

 of the rock. Each grain is surrounded by a narrow zone of ortho- 

 clase, which in its turn is surrounded by micropegmatite. Ferro- 

 magnesian constituents are very rare. They are represented by 

 biotite in this rock ; but in a similar rock known only in ejected 

 blocks, and formerly supposed to be a definite mineral (krablite), a 

 green augite occurs. 



It seems impossible to avoid the conclusion that the volcanic 

 rocks of Iceland and the West of Scotland have been derived from 

 the same magma-basin, and that the sequence has been the same 

 in both cases. 



