OPAL PSEtJDOMORPHS PROM WHITE CLIFFS ANDERSON AND .TEVONS. 37 



would have the effect of displacing the edges C and C ' towards 

 the cleavage lines crossing the edge B (PI. vii., figs. 8, 9), 

 and also of rendering the angle CC measured over the 

 apex more acute (PL vii., figs. 2, 3). Another series of 

 striations observed on the *• faces of other crystals, and 

 having a diiection nearly parallel to the edges C and C ' may be 

 due to the coming in of the m (110) face. The n faces are 

 strongly striated, and in some cases distinctly stepped, the direc- 

 tion of the strife and steps being parallel to the cleavage. These 

 must be due to oscillatory combination of u with C (001), or ti 

 with 71 (112) or v (113), any of which would make the angle be- 

 tween the edges A B more acute than it would be in a perfect 

 crystal (PI. vii., figs. 8, 9). 



We may now enquire what effect the oscillations described 

 would have on the normal angles. It is readily seen that by their 

 means the normal angle ss ' would be enlarged, and the angle n"n' ' ' 

 diminished, while the angles su would be either diminished or 

 enlarged according as the effects of the oscillation of (hkh) on s or 

 of (001) on n predominate. Now, from the mean values obtained 

 by measurement, it will be observed that the departure from 

 theory of the angles ss ' and n''n" ' is in the direction we should 

 have expected. The mean value for the angle hu was found to 

 be greater than the theoretical, which accords with our observation 

 that the oscillation on the n faces is frequently much more pro- 

 nounced than that on the s faces. 



Glauberite is commonly found in association with rock salt, 

 thenardite, mirabilite, and other sulphates, carbonates, itc, char- 

 acteristic of salt lake deposits. It is soluble in water, and can, 

 therefore, occur only in jDrotected places or in arid regions. Most 

 likely at White Cliffs it was formed in deposits of mud or ooze 

 and not directly from solution. The consequent interference with 

 the regular growth of the crystals may possibly account for the 

 cur\ature of the faces through oscillatory combination. It is 

 noteworthy that with the single exception of the thinolite of 

 Lake Lahontan all the pseudomorphs resembling the 8angerhausen 

 mineral, as also the pyramidal crystals of celestite from Virginia 

 described by G. H. Williams,'- which furnish the chief argument 

 for the celestite origin, have been found embedded in clay, mud 

 or marl. Thus it may be that the resemblance between 

 specimens from different localities, which after all consists mainlv 

 in the curved and tapering form, is to be referred rather to the 

 similar conditions of growth than to identity of species. 



1- Williams — Am«i-. Journ. Sci., xxxix., 1890, p. ls;3. 



