136 PEOCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 



large, clear, glass}^ crystals (microtiue of Tscbermak) of a feldspar simple 

 in structure, my analysis of wliicli proved it to be audesitc. Some of 

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

 ada, 1 8G3, Avere 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 fine 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 tl\3ir sections? the 

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

 do not, when the grains are too small to allow cleavage fragments to bo 

 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 clement 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 ej^isticity planes in chance sections are 

 liable to lead to wrong results. 



National Museum, ^pn7 L>0, 1881. 



OIV CERTAIIV CRETACEOUS FOSSiTff.S FROITI ARKANSAS AND C0£.0. 



RAOO. 



By C. A. ^WHITK. 



In volume III of the Proceedings of the United States National 

 Museum, pp. 157-1'>2, 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 Eock, and by him i)resented 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 sj)ecific and generic relations is not entirely 

 satisfactory in all cases. 



*Tliis volume, page — . The method of separating constitueuts of rocks by means 

 of a heavy solution wa? first proposed, according to von Lasauls, hy Fleuvian de 

 Bellevne and Cordier, at the beginning of this century. Church suggested the use of 

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

 Magazine in November, 1877. 



Thoulet 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 lluid to 3.2. (Inaugural Dissertation, Stuttgart, 1880.) 



