266 Mr J. D. Dana on the Origin of Trap Minerals. 



time to receive the waters and any depositions they might be 

 fitted imder the circumstances to make. 



We are led by these considerations to another point in 

 connection with this subject — the probable condition under 

 which the different amygdaloidal minerals have been formed. 

 Have they all proceeded from heated solutions, or all from 

 cold solutions I or can we distinguish some which are indubi- 

 tably of one or the other mode of formation % 



Bearing on these questions, we notice such facts as are 

 afforded by the condition and relative positions of the mine- 

 rals in geodes. And I would here acknowledge my obliga- 

 tions to the valuable memoir, before alluded to, by Messrs 

 Jackson and Alger. The paucity of information on this sub- 

 ject to be found in the various accounts of similar rocks by 

 other writers, is surprising. Even where special pains have 

 been taken to describe the mineral species, the relative posi- 

 tions of the minerals is very seldom noted. It has been 

 altogether too common among geologists to treat mineral 

 information with a degree of neglect almost amounting to 

 contempt, although, as facts will probably hereafter shew, 

 they lie at the basis of an important branch of geological 

 science. 



But to proceed with the subject before us. We find that 



Quartz or chalcedony, and datholite, very seldom overlie 

 other mineral species in geodes or amygdaloidal cavities, 

 while the latter often overlie them.* 



Prehnite is usually lowermost with reference to all the 

 species except the two just mentioned. Occasionally it is 

 found upon analcime, as at the Kilpatrick hills. 



Analcime is commonly situated below all, except quartz, 

 datholite, and Prehnite. 



Of the remaining species, chabazite, stilbite, harmotome, 

 Heulandite, scolecite, mesole, Laumonite. and apophyllite, it 

 is more difficult to distinguish an order of arrangement. My 



* The writer has observed stilbite, apophyllite, calc-spar, and Prehnite, over 

 lying datholite, and various species over Prehnite. 



