726 ABSTRACTS OF PAPERS 



mystery largely disappears when a comparative study is made of the stratig- 

 raphy of the Appalachians as a whole. The paper compares graphically the 

 stratigraphy of the southern and northern Appalachians and the Central 

 States with reference to the occurrence of oil and gas. 



GRANULARITY LIMITS IN PETROORAPHIC-MICROSCOPIC WORK 



BY FBED E. WRIGHT 



{Abstract) 



In this paper the petrographic microscope is treated as a measuring device 

 for the exact determination of the optical properties of crystal plates, espe- 

 cially of minute crystal fragments and of crystallites. The methods now 

 available for the purpose are considered briefly with special reference to their 

 accuracy and applicability to tlie investigation of fine-grained silicate prepa- 

 rations. Attention is directed in particular to the lower granularity limits at 

 which satisfactory measurements of the different optical properties of a min- 

 eral grain can still be made. 



ARKANSAS DIAMOND-BEARING PERIDOTITE AREA 

 BY L. C. GLENN 



{Abstract) 



Evidence was offered of the circulation of presumably thermal waters about 

 the margin of the original pipe described by Branner. A supposed extension 

 of the peridotite area proves to consist of disintegrated peridotite mixed inti- 

 mately with well-rounded quartz sand and occasional water-worn chert peb- 

 bles and was evidently water-laid. Indications of the age of this material 

 narrow down the period within which the extrusion of the peridotite must 

 have occurred. 



Discussion 



Prof. A. H. Purdue : The paper by Professor Glenn is of great interest to 

 me, for it appears to establish the age of the peridotites as post-Lower Cre- 

 taceous and pre-Upper Cretaceous, for recent work over the area by Mr. 

 H. D. Miser and myself had disclosed a marlved unconformity in the imme- 

 diate locality of the peridotite, between the Lower and Upper Cretaceous. 

 The material of the intermediate area described by Professor Glenn was in 

 all probability put down while the Upper Cretaceous shoreline occupied this 

 area, the material, of course, having been derived from the disintegration of 

 the peridotite. 



VARIATION OF THE OPTIC ANGLE OF GYPSUM WITH TEMPERATURE 



BY EDWARD H. KRAUS 



{Abstract) 



By using an oil bath to determine the variation of the angle of the optic 

 axes of gypsum at different temperatures, it is found that gypsum is optically 

 uniaxial at approximately 90 degrees Centigrade for sodium light Although 



