270 Reviews — Dr. M. E. Wadsicorth — Lithological Studies. 



The third chapter deals with the Peridotites, rocks of both 

 meteoric and terrestrial origin. In his introductory remarks, the 

 author expresses the opinion, in which most of those who have 

 studied the subject will agree, that the Peridotites are essentially of 

 one type, viz. rock composed of olivine with sundry adventitious 

 minerals, as magnetite and members of the spinel group, enstatite, 

 augite, etc. ; but inasmuch as varieties are generally admitted, he 

 accepts or proposes the following varietal subdivisions and names : 

 olivine mainly = Dunite : olivine + enstatite = Saxonite : olivine-|- 

 augite =: Picrite : olivine + diallage = Eulysite : olivine + enstatite -}- 

 augite = Buchnerite : olivine -|- enstatite + diallage := Llierzolite. In 

 regard to these subdivisions we must venture a slight criticism. Tlie 

 name Eulysite is generally (and we believe was first) applied (as 

 afterwards used by the author) to the peridotite from Thunaberg, in 

 which a fair proportion of garnet is present; this we take to indicate 

 a not unimportant difference in chemical composition. We doubt, 

 also the possibility of separating the peridotites with diallage from 

 those with augite. Our own specimens of Lherzolite, taken from the 

 typical localities, contain augite, and the diallage is not a strongly- 

 marked form. We should, therefore, prefer to give distinctive 

 names to the olivine-augite, and the olivine-enstatite-augite rocks 

 only, prefixing diallage as an epithet in cases where that vai'iety of 

 pyroxene happened to be well marked. If, however, it is con- 

 sidered desirable to maintain this distinction, which indeed may le 

 defended by the use of the term ' gabbro,' as distinct from dolerite, 

 then we think Lherzolite may fairly be retained for the augite- 

 bearing variety. Picrite also, we think, has more commonly been 

 applied to rocks in which the pyroxenic constituent dominates over 

 the olivine, and a little felspar is not seldom present — that is to an 

 augite- (or hornblende-) olivine rock, rather than to an olivine-augite. 



Dr. Wadsworth then passes on to describe a large number of 

 peridotites from various parts of the world, both terrestrial and 

 meteoric, as well as serpentines, the relations of which to peridotites 

 are clearly indicated. A discussion of the asserted olivine-schists of 

 the Troad is inserted here, which will be of use to the student after the 

 rash statements which have been made concerning these rocks. We 

 are not surprised that Dr. Wadsworth is convinced that the schists 

 are derived from the olivine-rock, and not the olivine-rock from the 

 schists ; the latter origin is opposed to such a number of facts, that 

 nothing but the clearest evidence could be held to substantiate it, 

 and this is certainly not forthcoming. The last chapter deals with 

 the basalts, but only those of meteoric origin are discussed here. 



Long and extremely valuable tables of chemical analyses follow ; 

 the first indicating the relation of chromite and picotite, the others 

 the complete analyses of a large number of the rocks discussed in 

 the preceding chapters. The plates illustrative of some types of 

 these are well executed, and will be found valuable. As no group 

 of rocks has been more maltreated by speculative geologists than the 

 peridotites. Dr. Wadsworth's volume will be a boon to the perplexed 

 student, and a valuable work of reference to every petrologist (or 

 petrographer?). T. G. B. 



