58 SCIENCE PROGRESS. 



rocks belonging to several distinct types. They are post- 

 Archsean but apparently pre-Cambrian, and seem to be in 

 part superficial lavas, in part volcanic dykes. They include 

 syenite-porphyry (a porphyritic trachyte), augite-porphyrites 

 (augite- and hypersthene-andesites), melaphyre (olivine- 

 basalt), and spilite (amygdaloidal andesite). Judging from 

 the descriptions and photographic figures given, these rocks 

 show only the ordinary changes, such as the conversion of 

 olivine to serpentine and hypersthene to bastite and the 

 filling of the vesicles by secondary products, all essential 

 characters being beautifully preserved. 



We may mention in passing the remarkable occurrence 

 of acid volcanic rocks, in a perfectly fresh condition, on the 

 shores and islands of Lake Mien in the south of Sweden, 

 where they occur surrounded by gneisses, granite, diorite, 

 etc. These rocks were discovered as blocks and in situ by 

 Hoist (9), and have been microscopically described by 

 Szadeczky ( 10). They include glassy and microspherulitic 

 rhyolites, rhyolitic tuffs and breccias, etc., and have not 

 suffered devitrification. In this case, however, there seems 

 to be no clue to the age of the rocks, and the eruption is 

 probably to be referred to no very distant epoch. 



Our knowledge of the older volcanic rocks is perhaps 

 less complete as regards the basic than as regards the acid 

 types, but Barrois (11) has given a valuable description of 

 a series of basic eruptions of Lower Palaeozoic age in 

 Brittany. The rocks, which we should term dolerites and 

 augite-andesites, with associated tuffs, are developed in the 

 Menez-Hom district in the department of Finistere, and 

 belong to several horizons in the Ordovician and Silurian 

 systems. The massive rocks occur partly as dykes and 

 intruded sills but mainly as undoubted coulees, and are 

 described as various types of diabases and augite-porphyrites. 

 Of these the former belong to the earlier eruptions, which 

 were submarine, while the latter are found especially 

 characterising the later eruptions, which were subaerial, the 

 differences between the two sets of rocks being due to the 

 different conditions under which they consolidated. It is 

 also noticed that the diabases occur only in thick Mows, 



