19 



GEOGNOSTIC OCCURRENCE. 



In the mountains of Passau, this fossil is found imbed- 

 ded in a coarse granular aggregate of felspar and horn- 

 blende, and felspar and actynolite; therefore belong- 

 ing to the genus green-stone, and order of primitive trap. 

 In Norway it occurs in rocks belonging to the same for- 

 mation, in which the celebrated layers of magnetic iron 

 ore lie, and is associated Avith hornblende, and several 

 individuals of a tribe not as yet sufficiently examined and 

 described, but which evidently constitute middle links be- 

 tween actynolite and hornblende, and to which the names 

 arendalite and acanticone have been applied. Near Dresden 

 and Briinn it is found dispersed through sienite; and at 

 Gahvay in Ireland, in an uncommonly beautiful porphy- 

 ritic sienite. Hence it appears, that this fossil has only, 

 occurred in rocks belonging to primitive trap, or in 

 sienite, the last crystallization which took place within the 

 primitive period, and must therefore be considered as a 

 later production than rutile. Here a consideration of the 

 laws of crystallization countenances the observations on 

 the order in which the primitive rocks follow one another. 

 The rutile, consisting of few and simple elements of co- 

 temporary origin, with a granite, in which rock crystal 

 occupies the place of quartz, and adularia that of com- 

 mon felspar, sufficiently bespeaks a period, when the so- 

 lution being purer and more tranquil, furnished an earlier 

 and purer crop of crystals ; while the confused and irre- 



D .2 gular 



