1 882.] Mineralogy. 421 



have been of comparatively small vertical extent, the elevation of 



mountain-chains being formed by lateral thrust. The March 



number of the American Journal of Science contains the second 

 of a series of articles upon the flood of the Connecticut River 

 valley from the melting of the Quaternary glacier, by J. D. 

 Dana. The average depth of this flood, taken from the level of 

 the wide terrace out of which the present river-bed is hollowed, 

 was 140 feet north of the Massachusetts line, and about 125 feet 



in Massachusetts and Connecticut. In the same magazine C. 



D. Walcott describes a new genus of the order Eurypterida, from 

 the Utica slate. As far as known no example of the Eurypter- 

 ida has previously been described from a lower horizon than the 



Medina sandstone. At a recent meeting of the Paris Academy 



of Sciences, M. Emile Blanchard stated that the condition of the 

 fauna and flora of New Zealand showed it to be a remnant of a 

 southern continent submerged during the modern epoch of the 

 earth's history. 



MINERALOGY. 1 



Pseudo-symmetry. — Much interest has been excited among 

 crystallographers in those curious crystalline forms, which, while 

 appearing to be simple forms belonging to one system of crystal- 

 lization are now regarded as composed of a number of twinned 

 crystals of another system. These are the crystals which ex- 

 hibit the " optical anomalies " for which so many explanations 

 have been offered. 



Some recent investigations in this direction appear to be over- 

 turning our most elementary mineralogical conceptions. Thus, 

 the garnet, for example, so constant in crystalline form, notwith- 

 standing the great variations in composition, has always been 

 considered a type of the isometric system. Yet, by means of 

 delicate 'optical investigation, the conclusion has been reached 

 that several of the varieties of garnet are not simple dodecahed- 

 rons, as indicated externally, but are composed of twelve ortho- 

 rhombic crystals symmetrically arranged around a central point. 

 It has moreover been stated that in the case of the varieties topazo- 

 lite and ouvarovite, each of these twelve orthorhombic crystals 

 are themselves composed of four more elementary crystals, mak- 

 ing a group of 48 crystals in all to produce each apparently 

 simple form. 



Pseudo-symmetrical crystals formed by a less number of twins 

 arranged around a line or plane have long been ' 

 three crystals often twinned in aragonite, the four ' 

 the six in witherite and the eight in n " 

 °f twins symmetrically placed around 1 

 n 'ngs m the plagioclase felspars offer an example of numerous 

 twins on a single plane. 



' Edited by Professor Henry Carvill Lewis, Academy of Natural Sciences, Phila- 



