18 GEOLOGICAL MEMOIRS. 



preliminary results, leaving to him and Dr. Haidinger the denomi- 

 nation of this new species. Dr. Haidinger chose the denomination 

 of Hoemesite, indicative of the friendly and scientific intercourse 

 between the discoverer and his former collaborator at the Imperial 

 Museum. 



Hoemesite ranks among the "Haloids" of Mohs's system, and 

 appears in spheroidal groups of crystals, developed within the free 

 interstices into small rhomboid lamellae, with an acute angle of 36° ; 

 which Dr. Haidinger regards as belonging to the Augitic (Mohs's 

 " Hemiprismatic") system. The crystals are white and flexible, 

 with a single cleavage-plane of pearly lustre, parallel to the longi- 

 tudinal surface. The hardness is equal, or perhaps inferior, to that 

 of talc (1-0 of Mohs's scale) : specific gravity=2'474. According 

 to Chev. Ch. de Hauer, the electro-negative element is arsenic acid, 

 and the chemical formula is 3MgO.As0 5 + 8HO (arsenic acid, 46-33; 

 magnesia, 24-54 ; water, 29-07 ; loss, 0-06). 



The only specimen at present known came to the Imperial 

 Museum with the celebrated collection of Yan der Null, and has 

 been mentioned by Mohs, in the description of this cabinet, under 

 the denomination of talc, to which, and especially to its variety 

 with stellar fracture (known as " pyrophyllite"), it bears indeed a 

 striking resemblance. This specimen is said to come from the Banat, 

 probably from the environs of Oravieza. The new species is particu- 

 larly interesting as filling up a blank in the series of the hitherto 

 known native arseniates of copper, iron, cobalt, and lime. 



[Count M.] 



On the Geology of Tahiti and Taiarapoo. By M. Kt/lczycki. 

 [Proceed. Imp. Geol. Instit. Vienna, December 13, 1859.] 



Dr. Scherzer sent to Director Haidinger a note from M. Kulczycki, 

 Director of the Native Department at Tahiti, on the geological 

 structure of this island and of the peninsula of Taiarapoo. The author 

 recognizes the existence of the three regions, distinctly marked out 

 by their botanical features, so well described by Dr. C. Darwin, 

 and undoubtedly essentially connected with the geological history of 

 the island and its gradual rise above the level of the sea. The two 

 crateriform systems of Tahiti and Taiarapoo belong to the first, or 

 eruptive period. During the second period the solidified crust was 

 upheaved to its present level, and torn into radial fissural valleys. 

 The level of the sea during the first period may probably still be 

 traced by means of a corallian girdle running round the whole island 

 below its present summits (3800 ft.). The fossil madrepores men- 

 tioned by Mr. Stutchbury (Sir C. Lyell's ' Geology ') may probably 

 belong to this girdle, the existence of which, however, may still 

 remain problematic for a long time, on account of the inaccessibility 

 of some part of the island. 



Basalt, compact, with olivine, or irregularly columnar, — porose 

 zeolitic lavas, — and trachytes (on the southern portion, and at the 

 east point of Taiarapoo), sometimes decomposed into impure kaolin, 



