88 Revieics — G. H. Williams'' s Gabhros of Baltimore. 



British geologists also. The author has minutely studied, in the 

 field and by microscopical and chemical analysis, the convei'sion of 

 a pyroxenic to a hornblendic rock. The rocks occur (Chapter I.) in 

 an oval district 50 square miles in area, breaking through the 

 gneissic rocks to the west and north-west of Baltimore. The 

 pyroxenic type {Hyper sthene-gahhro, Ch. II.) is a massive rock con- 

 sisting essentially of bytownite, diallage, hypersthene, and some 

 brown hornblende. The typical hornblendic rock (Ch. III.), which 

 the author names Gabhro-diorite, is either massive or schistose. It 

 contains green hornblende, often true uralite but in part compact, 

 and bytownite showing incipient saussuritization and also breaking 

 up into epidote near the contact of the felspar with adjacent 

 hornblende. 



Chapter IV. deals with the genetic relations of these two very 

 different rocks. In the field they are found to be inextricably 

 involved, and to graduate imperceptibly into one another. The 

 average analyses of the two agree closely, though considerable 

 variations of composition occur under each type. Microscopical 

 examination of the transition-zone reveals the nature of the changes 

 that have taken place. Both the diallage and the hypersthene of 

 the gabbro are seen to pass gradually into green hornblende, but in 

 different ways. The conversion of the diallage is apparently a true 

 paramorphosis, this mineral and the resulting hornblende giving 

 closely similar analyses. The hypersthene, on the other hand, for its 

 amphibolization, must take up alumina and lime from the felspar, 

 and in the earlier stages of the change the mineral is bordered at its 

 contact with the felspar by a double rim, the inner zone of fine 

 colourless fibres, the outer dark green and comparatively compact. 

 This is analogous to what has been observed when olivine passes 

 into amphibole. 



The evidence of the metamorphism of gabbro into diorite in this 

 district is complete at all points. The author regards augite and 

 hornblende as dimorphous, the former representing the stable form 

 of the molecules at high, the latter at low temperatures. Augite 

 being unstable at low temperatures will tend to revert to hornblende 

 under any influence which facilitates molecular motion. Pressure, 

 though a potent condition in many instances, is not regarded as 

 necessary. The concluding chapter is devoted to the description of 

 certain associated olivine-bearing rocks and serpentines. The chief 

 type of olivine rock is described as a felspathic peridotite : besides 

 olivine, bronzite and diallage, it contains up to five per cent, of 

 bytownite, partly zeolitized. The olivine and pj^roxenic constituents 

 exhibit the usual secondary actions, and the double rim of amphibole 

 is also found at the contact of the olivine and felspar. The structure 

 is remarkable, the olivine being the latest-formed mineral, in which 

 the pyroxenes occur porphyritically. The most abundant type of 

 serpentine in the district is one containing tremolite, probably 

 secondary after pyroxene. In some cases a further change of 

 amphibole into talc is observed. A. H. 



