142 CARNEGIE INSTITUTION OF WASHINGTON. 



certain places, however, and especially at the immediate contacts against 

 the limestone, the actual composition of the gneiss appears to be a direct 

 result of assimilation and no correspondence between Fe : Mg ratio and 

 "basicity" exists. The distribution of magnesia and ferrous oxide among the 

 different mafic minerals was found to show a certain regularity, the mica 

 always being richest and the clinopyroxene poorest in the iron compound. 

 The variation of the "Fe quotient" is believed to offer an important char- 

 acteristic of crystalline rocks, though at present little understood. 



When silica-bearing limestones are subjected to metamorphism, reactions 

 occur between the carbonates and silica, and silicates of lime and magnesia 

 are formed. The temperature of reaction varies with pressure and is different 

 for different minerals formed, as pointed out by V. M. Goldschmidt. The 

 writer's earlier investigations have established that, among the common 

 accessory silicates in limestones, wollastonite requires the highest tempera- 

 tures to form, and diopside and tremolite successively lower. At still lower 

 temperatures silica in the form of quartz remains uncombined. Thus we 

 may distinguish the following types of metamorphic limestone: wollastonite 

 limestone, diopside limestone, tremolite limestone, and quartz limestone. 

 These types may be used, under certain conditions, as a geologic thermometer, 

 and it is hoped that the equilibrium curves of the different silicates with the 

 carbonates may soon be determined experimentally. 



The limestones of western Massachusetts were found to represent all the 

 above-named types excepting the wollastonite limestone. Their mode of 

 occurrence harmonizes with the writer's earlier experience, diopside limestone 

 occurring at the immediate contacts of the gneiss, and tremolite limestone 

 and quartz limestone successively farther away. 



A review of the writer's experience from limestone-bearing regions where 

 intrusive granites occur seems to prove that such phenomena of assimilation 

 of limestone as those observed in western Massachusetts are not at all of 

 regular occurrence. Preferably they seem to occur in those regions where 

 gneiss magmas have been intruded in connection with mountain folding, 

 thus in all probability being dependent on the mechanical conditions. It 

 appears, also, that assimilation does not require very high temperatures, 

 being a common phenomenon in granite pegmatite cutting limestones. 



(448) Nephrite celt from Bahia, Brazil. Henry S. Washington. Pan-American Geologist, 



37, 198-202. 1922. 



A small celt of nephrite from the Paraguassu River, in Bahia, is described. 

 Nephrite objects from Brazil are very rarely seen outside of that country. 

 The nephrite is of the typical color, luster, toughness, and micro-structure. 

 It is composed entirely of a pale greenish tremolite in very fine fibers. The 

 optical characters, density, and a chemical analysis are given. It is compared 

 with other nephrites from Brazil. It is suggested that one of these, of unusual 

 chemical composition, may be a mixture of albite and enstatite-diopside, 

 analogous to the mixture of albite and diopside-jadeite found in Central 

 America and Mexico. 



(449) The crystallographic and atomic symmetries of ammonium chloride. Ralph W. G. 



Wyckoff. Am. J. Sci., 3, 177-183. 1922. 



It is shown that not only is the symmetry of the structure that has been 

 assigned to ammonium chloride in conflict with its observed symmetry, but 

 that there is no other possible structure which will possess the requisite sym- 

 metry. As a result the necessity of further study of the crystallography of 

 this salt is urged. 



