412 THE JOURNAL OF GEOLOGY. 



had been placed in Professor Lessen's hands before his death, while 

 communications on other special groups are doubtless to be expected. 



These investigations naturally suffer from the forced absence of all 

 field observations on the part of their authors, but the purely petro- 

 graphical study of the material brings to light many points of interest, 

 while it furnishes the only sort of detailed information regarding the 

 rocks of these remote regions which we can for the present hope for. 

 It is here desired only to direct attention to a few of the most striking 

 results obtained from the Brackebusch material by the three authors 

 last cited. 



Dr. Kiihn's paper on the crystalline schists treats principally of 

 gneiss, and offers little that is new. It is mostly occupied with addi- 

 tional evidence of structural and chemical changes due to dynamic meta- 

 morphism in the sense of Lehmann. The most noteworthy of these 

 are development and microstructure of fibrolite ; production of augen - 

 gneiss from porphyritic granite ; development of microcline structure 

 in orthoclase by pressure ; secondary origin of microcline, micro- 

 perthite and micropegmatite ; alteration of garnet to biotite and horn- 

 blende. 



Dr. Sabersky's paper on the coarse-grained granites or pegmatites 

 is entirely mineralogical, and is devoted principally to elucidating the 

 structure of microcline. The author concludes that the well-known 

 gridiron structure is due, not to two twinning laws (the Albite and 

 Pericline), as has been generally supposed, but to the Albite law alone, 

 in accordance with which the individuals form both contact and pene- 

 tration twins, like the albite crystals form Roc- tourne, described by G. 

 Rose. 



Dr. Romberg's paper on the Argentine granites is much more 

 extensive than the two preceding. It is embellished by seventy - two 

 microphotographs, many of which admirably illustrate the special 

 points described. He comes to several results of great petrographical 

 significance, the most important of which relate to the origin of quartz - 

 feldspar intergrowths in granitic rocks. He clearly shows that beside 

 the original granite quartz there is also much of a secondary nature 

 present. This is not microscopically distinguishable from the original 

 mineral, but its later genesis is demonstrated by many careful observa- 

 tions on its relation to other constituents. The abundant secondary 

 quartz is regarded- as the product of weathering — principally of the 

 feldspar, into which it has a peculiar tendency to penetrate. The 



