54 SCIENCE PROGRESS. 



poor in nepheline and approaching the trachytes in 

 character. 



It is to be expected that further examination will dis- 

 cover lavas of these various types in other parts of Britain. 

 Watts has already noted in the Limerick district a lim- 

 burgite lava, also of Carboniferous age, and Hobson (4) has 

 described from a neighbouring locality the allied type 

 augitite, in which olivine as well as felspar is wanting, the 

 rock consisting essentially of two generations of augite and 

 magnetite with some base which has probably been glassy. 



Meanwhile individual geologists in Germany and other 

 countries have recognised the importance of secondary 

 changes, such as devitrification, in discussing the characters 

 of the older volcanic rocks, and much light may be expected 

 to be thrown upon the various "porphyries," " cerato- 

 phyres," " porphyrites," " meiaphyres," " schaalsteins," etc., 

 when they come to be compared, from this point of view, 

 with recent lavas. Some of the researches of this kind 

 already published have a special interest as referring to 

 what may be regarded as classical districts for geologists. 



In this connection we may profitably notice Sauer's (5) 

 work on the pitchstones and porphyrites of the Meissen 

 district in Saxony. These form respectively the lower and 

 upper parts of a series of lavas, both underlain and overlain 

 by tuffs, and Sauer's survey conclusively establishes the 

 true volcanic nature of the whole. The data are wanting 

 to fix their precise age, but they almost certainly belong to 

 a late epoch in the Palaeozoic. The well-known "pitch- 

 stones" are acid rocks containing only scattered crystals in 

 a ground essentially of glass. Minute granules of black 

 iron-ore occur in the glass, sometimes aggregated in rows. 

 Large spherulites are met with in one type. Perlitic cracks 

 are universally present. A feature which several observers 

 have noted is the frequent occurrence in the glassy ground 

 of irregular, cloudy or " microfelsitic " patches. Sauer points 

 out the relation of these patches to the perlitic cracks, a 

 clear evidence of their secondary origin, and regards them 

 as due to the devitrification of original glass. By the 

 spreading of this change, the whole of the glassy ground 



