58 J. D. Dana on the Minerals of Trap 



mon among geologists to treat mineral information with a 

 degree of neglect almost amounting to contempt, although, as 

 facts will probably hereafter show, 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 spe- 

 cies 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 

 investigations only enable me to state that chabazite is usually 

 covered by the rest (when associated with them), yet it is 

 sometimes superimposed on stilbite; and apophyllite is almost 

 uniformly above all with which it may be associated ; calc-spar 

 is at different times above and below. We thus arrive at the 

 following as the usual order of superposition : — 



1. Quartz. 



2. Datholite. 



3. Prehnite. 



4. Analcime. 



5. Chabazite, harmotome. 



6. Stilbite, Heulandite, scolecite, natrolite, mesole, Lau- 

 monite, apophyllite. 



It is a reasonable inference that the species which covers 

 the bottom of a cavity was first deposited, and, as a general 

 rule, that the others above were formed, either simultaneously, 

 or in succession upon the lowermost, as their order may indi- 

 cate. Each is usually perfect in its most delicate crystalliza- 

 tions, so that we cannot suppose that the minerals first de- 

 posited often underwent change after their deposition, though 

 instances of this may no doubt be detected. 



It is also evident, that if there were any species formed pre- 

 vious to the complete cooling of the rock, or if any require 

 for their formation an elevated temperature, they are those 

 first deposited — the first in the above series. A few considera- 

 tions will place this, if possible, in a clearer light. 



Quartz, as we have stated in a preceding page, and fully 

 remarked upon elsewhere, enters largely into solution during 



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

 overlying datholite, and various species over prehnite. 



