REVJEWS 301 



bring out clearly the complex relations of the several members of the 

 system. 



Two distinct classes of gneissic rocks are recognized in the areas 

 examined, in addition to the numerous intrusions of igneous rocks of 

 various kinds. To each of these classes a collective name has been 

 assigned, taken from a locality where it is well exposed; the first 

 being known as the Rensch Gneiss and the second as the Schapbach 

 Gneiss. A few of the most notable varieties of each class of rock are 

 distinguished in mapping, but no attempt is made to map separately 

 the bewilderingly numerous and minute petrographical variations of 

 the gneissic rocks attempted in the survey of Saxony. The Rensch 

 gneisses consist chiefly of orthoclase, biotite, and. quartz. The mica 

 is usually abundant, and sillimanite is a characteristic accessory con- 

 stituent, often occurring as a paramorph after andalusite. Garnet and 

 sphene are seldom found. The rock often shows in the arrangement 

 of the constituents a structure similar to that seen in the hornstones of 

 contact zones. The presence of small lenticular segregations of quartz, 

 or of quartz and orthoclase, scattered through the rock is also a char- 

 acteristic feature of the gneisses of this class. Conformably inter- 

 banded with these Rensch gneisses are subordinate masses of quartz 

 schist, graphitoid schists and gneisses, pyroxene gneisses and amphib- 

 olites. 



The Schapbach gneisses are much more uniform in character, 

 usually poorer in mica, and have a marked tendency to assume a 

 granitic aspect, often passing over into granulites. Quartz lenses and 

 fine-grained, highly quartzose bands are absent, but garnet and orthite 

 are frequently present. The only inclusions found in the gneisses of 

 this class consist of a gabbro-like amphibolite. The gneisses of the two 

 classes sometimes seem to pass into one another along the contacts, 

 but the distinction is usually sufficiently well marked to enable them 

 to be properly separated in mapping. 



Although no direct expression of opinion concerning the origin of 

 these gneisses is given in the publications in question, it seems to be 

 the opinionof the survey that the two classes probably differ in origin, 

 the Rensch gneisses representing highly altered sediments and the 

 Schapbach gneisses being of igneous origin. The chemical evidence 

 afforded by a number of analyses of typical gneisses of each series 

 which are given, tend to support this view as does also the structure of 

 the rocks, and the character of the subordinate intercalated masses in 



