ANCIENT VOLCANIC ROCKS. 57 



Miigge makes a minute study of the fragmental rocks 

 associated with the lavas. Some of them are pure tuffs 

 consisting largely of ash-particles, which often show the 

 characteristic concave outlines suggestive of comminuted 

 pumice. In others, which the author styles " tuffites," 

 the ashy particles are mixed with ordinary sedimentary 

 material. The paper, illustrated by seven plates of photo- 

 graphic figures, is a valuable addition to our knowledge of 

 the older lavas. 



The Swedish geologists have in recent years described 

 various ancient lavas in every way comparable with the 

 rhyolites, etc., of Tertiary volcanic districts. Such, accord- 

 ing to O. Nordenskjold (7), are the " halleflintas " which 

 form part of the Archaean formations in the south-east of 

 Sweden. He separates them from the ore-bearingsedimentary 

 "halleflintas" of Central Sweden, and shows that many of 

 them have " characters in which they agree with the 

 ancient English felsitic rocks, which have been described by 

 the authors of that country as, in part, rhyolites and devit- 

 rihed obsidians". The correspondence is indeed very close, 

 and is only occasionally obscured by a certain schistosity 

 due to crushing. The rocks are usually porphyritic with 

 •crystals of plagioclase, orthoclase, and less frequently quartz. 

 The ground-mass varies from cryptocrystalline to some- 

 what coarsely crystalline, and frequently shows fluxion and 

 banding, eutaxitic structure, microspherulitic portions, and 

 other characteristic features of the acid lavas. In some of 

 the rocks a cryptocrystalline ground-mass shows regular 

 perlitic cracks occupied by secondary minerals, besides 

 trichites, margarites, and grouped crystallites : these are 

 obviously devitrified obsidians. Very interesting are the 

 so-called "conglomeratic halleflintas ". The supposed pebbles 

 are found to be really altered large spherulites with con- 

 centric shell-structure and central space occupied by calcite, 

 quartz, etc., and the description corresponds identically with 

 those of the "nodular felsites "' or altered coarsely spheru- 

 litic rhyolites so well known in North Wales. 



In Central and Littoral Sweden and across the Gulf ol 

 Bothnia, in Finland, Hogbom (8) has recorded volcanic 



