136 PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 



large, clear, glassy crystals (microtiue of Tschermak) of a feldspar simple 

 in structure, my analysis of which proved it to be andesitc. Some of 

 the anorthosites described by Dr. T. Sterry Hunt in the Geology of Can- 

 ada, 1 8G3, were proven by his analysis to be composed of pure labradorite, 

 and some sections of the same which he submitted to me for examination 

 were found to be composed of a multitude of small grains, none of which 

 were twinned. Some of the tine crystals of oligoclase from Bodenmais 

 are simple crystals so far as the ordinary mode of twinning is concerned. 



If feldspar habitually showed their cleavages in their sections? the 

 optical method might still be followed with some certainty, but as they 

 do not, when the grains arc too small to allow cleavage fragments to be 

 obtained for optical examination, the method followed by me* in the 

 examination of the feldspathic constituent of the Triassic diabase is the 

 most reliable. ' , 



In consideration both of the complexity of the feldspathic element in 

 most rocks, and of the possibility of the simplicity of structure in tri- 

 clinic feldspars, the very carefully developed methods founded upon the 

 relation of twinning planes and elasticity planes in chance sections are 

 liable to lead to wrong results. 



National Museum, April 20, 1881. 



OIV CERTAIIV CRETACEOCrS FOSSII.S FROM ARKAIVSAS AND COK.O- 



RAOO. 



By C. A. WHITE. 



In volume III of the Proceedings of the United States National 

 Museum, pp. 157-1G2, five species of Cretaceous fossils (together with 

 some Tertiary species) were described, but not then illustrated. Illus- 

 trations of those Cretaceous species are now given on the accompany- 

 ing plate of this volume, together with those of two other Cretaceous 

 forms which are for the first time described in this article. 



The Arkansan species were collected by Mr. E. O. Ulrich in the 

 vicinity of Little Rock, and by him presented to the Museum, together 

 with a parcel of other fossils, mainly mollusca, which he found asso- 

 ciated with them. The greater part of these Arkansan specimens are 

 in the condition of mere casts of the interior of the shells, and therefore 

 the determination of their specific and generic relations is not entirely 

 satisfactory in all cases. 



*This volume, page — . The inetliod of separating constituents of rocks by means 

 of a heavy solution wa? first proposed, according to von Lasaulx, by Fleuvian de 

 Bellevue and Cordier, at the beginning of tbis century. Church suggested the use of 

 the solution of the iodide of potassium in iodide of mercury, in the Mineralogical 

 Magazine in November, 1877. 



Thoulct bettered the method and introduced improved apparatus. (Bulletin de la 

 Soc. Mineral, de France, 1879, No. 1.) Victor Goldsclmiidt succeeded in increasing the 

 special gravity of the fluid to 3.2. (Inaugural Disserta.tion, Stuttgart, 1880.) 



