302 REVIEWS 



the case of the Rensch Gneiss. The evidence one way or the other 

 will, however, be greatly extended as the mapping with concomitant 

 chemical investigation progresses, and the Director of the survey. Pro- 

 fessor Rosenbusch, evidently desires to await this further evidence 

 before making any decided statements concerning the genetic relation- 

 ships of the complex. If the Rensch gneisses prove to be altered 

 sedimentary rocks their high content of feldspar and the presence in 

 them everywhere of lenticular masses and strings of quartz and feldspar 

 will certainly be cited by the French authorities as evidences of "grani- 

 tization." But two questions remain to be decided — first, whether the 

 high content in feldspar is not due to a high content of alkalis in the 

 original sedimentary rocks, these having been perhaps of the nature of 

 feldspathic sandstones, arkoses and greywackes, and secondly, whether 

 the strings and lenses of quartz or quartz and feldspar do not fill 

 spaces opened by the dynamic movements to which the rocks have been 

 subjected, quite independent of any granitic intrusion. Whether in 

 fact any mysterious cementation-like transfusion of granitic material 

 through these rocks has really taken place. The detailed chemical work 

 which is now being carried out will, when completed, undoubtedly 

 decide whether the supposed altered sediments have or have not a 

 composition which can be attributed to a sedimentary series. 



A similar twofold origin is claimed by Klemm for the crystalline 

 Grundebirge of the Spessart, although here the sedimentary portion is 

 believed to be of late Palaeozoic age and is possibly equivalent to a 

 series of schistose hornstones, graphite schists and garnet rocks, quite 

 distinct from the gneissic series of the Black Forest, which were found 

 by Andrese and Osann in the Odenwald to the north of Heidelberg. 



These studies bearing upon the vexed question of the origin of the 

 crystalline schists have at present an especial interest for petrographers 

 in America, where such enormous areas of these rocks are now under 

 investigation. ' Frank D. Adams. 



Glaciers of North America, a Reading Lesson for Students in Geog- 

 raphy and Geology. By Israel C. Russei l. Boston : Ginn 

 & Co., 1897. 

 The preparation of a work of this high grade by a busy university 

 professor of large professional experience and demonstrated investi- 

 gative ability, as a reading lesson for students of geography and 



