62 JOURNAIv OF THE 



time the small spherical concretions are being formed 

 about minute nuclei. Chalcedony often encloses or- 

 o-anic forms and so perfectly that the colors of the 

 plants thus encased are preserved. ^ After a great 

 lapse of time let all the calcareous matter of the de- 

 posit be replaced b}^ silica, and then follow a period of 

 pressure and uplift and you have the form as it occurs 

 to-day. 



That solutions pass through chalcedonic envelopes is 

 shown in water geodes and in geodes containing bitu- 

 men. ^ In this view we would expect the cast of silica 

 in the shell of chalcedony to be purer than that which 

 surrounds these forms. This is observed to be the case 

 with pala^otrochis. (p. 5\-). And from the rule of re- 

 placement by silica (pp, 60, 61) we would be surprised 

 to find internal organic structure. 



It is not the purpose of this paper to assign these 

 forms to any class or order, not even to show whether 

 they are animal or vegetable. But in passing we may 

 note certain classes of organisms, to one of which it 

 may be referred at some future time. 



(1) It might have been a calcareous sponge whose 

 spicules were destroyed by replacement. Though as 

 no spicules have yet been found, it can not be put down 

 as a sponge. 



Figure 16 represents a sponge similar in form to the 

 pakeotrochis.^ 



(2) It may belong to the class hydro2;oa. 



1. Transactions of the Geolog-ical Society', Vol. II. First Series. 

 p. 510. 

 D 2. Dynamical and Structural Geolog^y, W. O. Crosby, p. 275. 



3. Ward's Catalof,'-uc. p. 205. For description see Transactions of 

 the deolog-ical Society, Vol. 1. ]). 337. 



