Johnson and Warren — Geology of Rhode Island. 19 



I II Hi 



Calcu- 

 lated to 

 Eatio 100$ Kaiserstuhl 



Si0 2 37-16$ -619 -619 37%3 36-72 



Ti0 2 -07 



Fe 2 3 -12 



FeO 31-38 -436^1 31*5 29-96 



MnO -40 '005 ! _ on 



CaO tr. \ J '"- U 



MgO 31-16 -779 J 31-2 31-99 



Feldspar ) 



Insoluble f "" " ' 3 4 100-0 98-67 



100-63 



The ratio RO : SiO fl = 1'97 : 1'0, or very nearly 2 : 1. 



The olivine of the rock is, therefore, a hyalosiderite, or iron 

 rich variety, and corresponds very closely in composition to 

 that of a hyalosiderite (see column III above) given by Dana 

 (p. 453, Analysis ISTo. 32), from a basalt described by Rosen- 

 busch from the Kaiserstuhl.* The specific gravity of the 

 latter is given as 3*566, which is considerably lower than that 

 of the one just described. An estimate of the specific gravity 

 of an olivine of this composition based on the specific gravity 

 of fayalite, fosterite and certain common chrysolites, gave 3'70, 

 which is in good agreement with the value found experiment- 

 ally in the present investigation. It would therefore seem 

 that 3*7 represents the true specific gravity of an hyalosiderite 

 of about the above composition. 



The Crystallisation of the Rhodose.\ — The unusual mineral 

 composition of the rhodose and its peculiar texture raise many 

 interesting questions regarding the manner of its crystalliza- 

 tion from a molten condition which are deserving of careful 

 consideration. The cumulophyric arrangement of the feldspar 

 and the undoubted inclusion of crystals of the latter within 

 the olivine indicate that the feldspar was the first mineral to 

 separate from the magma. In order for the feldspars to have 

 attained so large a size and to segregate into isolated groups, a 

 considerable degree of molecular mobility is demanded, which 

 could have hardly obtained if the surrounding minerals, con- 



* Jahrb. f. Min., etc., p. 50, 1872. 



f The writer is fully aware that he is not in possession of the thermal data, 

 and other physical constants, nor of a knowledge of the possible equilibrium, 

 conditions that might exist between the various components of the magma, 

 so essential for an exact and satisfactory discussion of its consolidation. 

 The purely petrographical evidence in the present case is of such a character, 

 that notwithstanding an insufficient basis, a guarded discussion of the prob- 

 lem will be entered into, believing that it may contribute something of value 

 to the more general problem of rock crystallization. 



