GEOPHYSICAL LABORATORY. 145 



and the slag show that they are practically identical in composition, except 

 that the fused part or slag has lost nearly all its water, all its CO2, and about 

 one-half its sulphur, and has received a considerable contribution of iron 

 from the drill-pipe. 



Thermal tests in the laboratory show that a temperature nearly if not 

 quite as high as 1150° C. is necessary to produce the results actually obtained 

 in drilling. A metallographic study of the steel shows that the temperature 

 attained was 1050° C. or higher. 



The chemical composition and the microscopic characters of the material 

 fused show that it was of the nature of an arkose, a variety of sedimentary 

 rock which appears to be particularly susceptible to such fusion. Since the 

 rock was an arkose, the slag shows only a moderate departure in composition 

 from an igneous rock, but the difference is none the less real, and is in the 

 same direction as that exhibited by typical shales. On account of the arkosic 

 nature of the sediment, the general chemical similarity of the slag and igneous 

 material can not be regarded as affording any support to the theory that 

 igneous rocks are formed by the fusion of shale, nor do the thermal results 

 favor the view that remelting of shale could occur at moderate depths in the 

 earth's crust (say 6 to 10 miles) unless the normal temperatures prevailing 

 at such depths are notably augmented by intrusion of igneous matter from 

 greater depths. 



(507) The formation of interference figures. A study of the phenomena exhibited by trans- 

 parent inactive crystal plates in convergent polarized hght. F. E. Wright. 

 J. Opt. Soc. Amer., 7, 779-817. 1923. 



Of the many phenomena observed by the student of crystal optics, the most 

 impressive are interference figures. Experience has proved that in order for 

 him to grasp the special relations involved and to maintain a clear picture of 

 them, it is advisable for him to adhere to a single surface of reference and to 

 derive from it the several relations presented by crystals in polarized light. 

 For the petrologist whose work is with thin sections of rocks, and who is there- 

 fore chiefly interested in refractive indices and wave normals and not in ray 

 directions, the index ellipsoid is the simplest surface of reference. With the 

 aid of the curves of intersection of the index ellipsoid and concentric spheres 

 the standard equations required by the petrologist can be derived more 

 easily than by other methods. Moreover, the spacial relations are clearly 

 represented graphically by these curves and others derived from them. Thus 

 the equivibration curves give the positions in space of all vibration directions of 

 given length; the equirefringence curves, the positions of all wave normal direc- 

 tions of given refringence; the equibirefringence curves, the positions of all 

 directions of wave normal propagation of given birefringence. For a given 

 crystal plate it is possible with the aid of the orthographic projection of the 

 equirefringence curves to ascertain the positions of the zero isogyres in the 

 interference figure exhibited by the plate. This was first shown by F. Becke, 

 who named the orthographic projection curves skiodromes. With the aid of 

 the equibirefringence curves the isochromatic curves of the interference figure 

 can be derived graphically. For the student thus to ascertain graphically the 

 positions of the isogyres and the isochromatic curves of the interference figure 

 of a given crystal plate of definite thickness and optical orientation is highly 

 desirable because it affords him an insight into the phenomena presented by 

 crystal plates under the petrographic microscope. These conceptions have 

 proved useful in a critical study of the methods which have been proposed by 

 Professor Becke and by the writer for the measurement of the optic axial angle 

 when only one optic axis is in the field of view. The results of this study show 

 that neither method is theoretically correct; that of Professor Becke, because 



