GEOPHYSICAL LABORATORY. 139 



groups replace the calcium atoms and if NH 4 groups are introduced in place 

 of the fluorine atoms. This determination was made by and serves as an 

 illustration of the general method (previously discussed) for the study of the 

 structure of crystals which is based upon the theory of space-groups. Except 

 in attempting to place the chlorine atoms with accuracy, when it was assumed 

 that atoms scatter X-rays in an amount which is roughly proportional to their 

 atomic numbers and that in a lattice arrangement of atoms the intensities 

 of reflection follow qualitatively the order of l/(h 2 +k 2 -{-l 2 ), the only assump- 

 tion made that is not required in the ordinary determination of the wave- 

 length of X-rays from a reflection spectrum was that the four hydrogen 

 atoms of the ammonium radical are exactly alike. 



(438) The jade of the Tuxtla statuette. Henry S. Washington. Proc. U. S. Nat. Mus., 



vol. 60, art. 14. 1922. 



This "jade" statuette, which is now in the U. S. National Museum, is a 

 work of early Mayan art and is the earliest dated object of American art 

 (98 B. C). The material was studied chemically by the author and optically 

 by H. E. Merwin and the density was determined by L. H. Adams. It is 

 shown to be a diopside-jadeite, that is, an equimolecular mixture of the 

 molecules of diopside and soda jadeite, the mixture being a rather unusual 

 variety of jadeite. It is also shown that most of the Mexican and Central 

 American jadeites are more or less of this variety and that they differ in this 

 respect from the jadeites of southeastern Asia (Burma and Tibet). Further 

 study of the American jadeites is contemplated. 



(439) The development of pressure in magmas as a result of crystallization. George W. 



Morey. J. Wash. Acad. Sci., 12, 219-230. 1922. 



It has been shown that when a system composed of volatile and non-volatile 

 components, such as water and KN0 3 , is cooled, crystallization will take place 

 at a temperature lower than the freezing-point of the pure non-volatile salt 

 by an amount corresponding to the amount of volatile material present, and 

 that the corresponding three-phase pressure increases rapidly as the tempera- 

 ture is lowered from the melting-point of the salt. This increase is rapid, 

 whether measured in terms of the decrease in temperature of the three-phase 

 equilibrium or in terms of the content of volatile material in the solution. 

 From the latter fact it follows that in systems of the type of magmas in which 

 the non-volatile material is composed of such material as the silicates, and 

 in which the pressure required to retain any considerable proportion of water 

 in solution must be large, a comparatively small amount of crystallization 

 will result in a large increase in pressure. When a magma containing water 

 cools, with consequent crystallization and development of high pressure, 

 under an incompetent crust, a release of pressure will take place, which may 

 be catastrophic in violence or take the form of a succession of mildly explosive 

 outbursts. In case the magma cools under a competent crust, the pressure 

 will rise to a maximum and then decrease, probably without at any time 

 showing critical phenomena. 



(440) Chemical aspects of volcanism, with a collection of the analyses of volcanic gases. 



E. T. Allen. J. Franklin Inst., 193, 29-80. 1922. 



The original volcano gases can be traced back to the igneous rocks. Water 

 is probably the most important of the gases. The original volcanic gases 

 are generally changed in composition by the time they reach the point of 

 collection; in some cases they probably lose a portion of the strong acid 

 gases; in most cases they become diluted by steam from surface water and 

 by the atmospheric gases. 



In some volcanic emanations the relations between the inert gases point 

 to an atmospheric origin for these particular constituents; in others the inert 



