102 re;ports of investigations and projects. 



differ— at least, this is true of kupfferite and beryl— they do not give true 

 equilibrium with water-vapor at low pressures, while the zeolites under simi- 

 lar conditions do so (Friedel). Diopside and tremolite seem to give off their 

 water continuously, but not indefinitely, with rising temperatures, though it is 

 quite possible the curves represent cases of "false equilibria." 



Recent analyses indicate that all the amphiboles contain water. Actinolite, 

 glaucophane, and pargasite contain 1.3 to 3 per cent, mostly retained above 

 100°. The hornblendes also contain water, though usually in smaller quan- 

 tity. These facts, taken in connection with the above work on tremolite and 

 kupfferite, lead to the suspicion that the amphiboles generally contain dis- 

 solved water as a characteristic constituent, and are solid solutions. 



(S) On the measurement of extinction angles in thin sections. Fred. Eugene Wright. 

 Amer. Jour. Sci. (4), 26, p. 349, 1908. 



The measurement of extinction angles of minerals in the thin section is one 

 of the most common methods of petrographic microscope practice, and at the 

 same time one of the least satisfactory when accurate results are desired. It 

 is an exceedingly easy matter to measure, with one trial only and on favor- 

 able sections, extinction angles with a probable error of d= 1° to 2°, but to do 

 so within ± 10' is a very different matter. The methods in use for this pur- 

 pose may be grouped into two classes — those of general application and those 

 of limited applicability. The first class may again be subdivided into two 

 subclasses, either (i) the crystal is revolved between crossed nicols about its 

 position of total extinction, or (2) the crystal remains stationary, and the 

 accuracy of its position for total extinction tested by revolving the upper nicol 

 or by inserting one of several different optical devices to increase the sensi- 

 tiveness of the test under prescribed conditions of illumination. These devices 

 include the Calderon ocular, the Bertrand ocular, the Bravais-Stober plate, the 

 Traube plate, also twinned plates and wedges of selenite, artificially twinned 

 plates and wedges of quartz, the circularly polarizing bi-quartz wedge-plate 

 and the bi-nicol ocular. Of these devices the last two are the most universal 

 and can be so used under any given conditions of illumination that the phe- 

 nomena observed are the most sensitive which it is possible to attain by de- 

 vices of this type. 



On comparing the relative sensitiveness of the different methods under 

 the same conditions, it is found that the method of testing the position of total 

 extinction for the crystal by revolution of the upper nicol is, on colorless min- 

 eral-plates, at least twice as sensitive as that of simply turning the crystal to 

 its position of apparent maximum darkness under crossed nicols. Under the 

 same conditions the methods requiring the use of one of the several plates or 

 wedges mentioned above are at least four times as sensitive as the ordinary 

 method. With the exception of the last two devices, however, these different 

 plates do not furnish equally sensitive results for the different conditions of 

 illumination which may arise. In accurate work adjustable sensibility is of 

 prime importance, particularly if a given device is to be of general applica- 

 tion. These requirements are best filled by the bi-quartz wedge-plate, by 

 means of which the angle of rotation can be varied from 0° to any desired 

 angle. The two halves of this wedge rotate in opposite directions, and on in- 

 sertion that angle of rotation can be secured for which the contrast in the 

 intensity of the halves of the field is most striking for a slight deviation of the 

 •crystal from its true position of extinction. 



