Reviews — Guides to Pomerania and Bornholin. 569 



works towards the side in such places where motion is retarded and 

 ■carrying power reduced. In this respect they conform exactly to 

 the action of rivers which deposit material in their inner bends. 



Rock trains may appear suddenly in the middle of a glacier or 

 ■at the junction of two streams. The first are undoubtedly caused 

 by the erosion of subglacial spurs or crags. Those formed at the 

 point of union of two glaciers are usually regarded as being formed 

 by the joining together of two lateral rock trains. 



There are cases, however, where rock trains are formed at the 

 junctions of glaciers, and no lateral rock trains fringe the tributary 

 glaciers. In front of the rocky islands or spurs which separate 

 the glaciers at the point of confluence, a hollow is always seen in 

 which a lakelet often exists. This is the counterpart of the hollow 

 on the down-stream side of a river after passing under a bridge 

 supported by piers. 



Objects carried by rivers tend to accumulate in this hollow, and 

 may linger there a long time before they join the main current 

 and get carried away. 



Thus rock trains may be formed by debris being thrust out of 

 glaciers at similar places where motion is small. In these instances 

 the fragments are probably torn off under the ice from the flanks 

 of the dividing spurs, and they may be compared with those 

 originating in the middle of a glacier. 



S, E "V I DB -W" S. 



I. — Geological Guides to Pomerania and Bornholm. By 

 W. Deecke. " Geologischer Fiihrer durch Bornholm."- 8vo ; 



130 pp. and map. " Geologischer Fiihrer durch Pommern." 8vo ; 



131 pp. Nos. 3 and 4 of Sammlung Geologischer Fiihrer. 

 (Berlin, 1899 : Gebr. Borntraegger.) 



DR. E. GEINITZ'S guide to Mecklenburg and that of Dr. Beck 

 to the section of Saxon Switzerland south of Dresden have 

 already given a favourable introduction to Borntraegger's series of 

 geological guides. The two next volumes in the series are worthy 

 of their predecessors. They are practical in method, clear in style, 

 and while they are written in language sufficiently popular to be 

 ■understood by any intelligent tourist, the information is sufficiently 

 precise and definite to give a geologist a clear idea of the geological 

 features of the districts described. Both guides are on the same 

 plan. They begin with a brief account of the topography of the 

 districts ; this introduction is followed by a longer general sketch of 

 the stratigraphical geology. The reader, having thus been told what 

 there is to be made out in the district, is informed exactly where to 

 find and how to interpret the available evidence. The books close 

 with a list of the more important geological works on each district 

 and with a full index. 



• The two districts, though not far distant geographically, are 

 very distinct geologically. Bornholm consists of a granite massif, 



