Sand Formation of the United States. 247 



woody fibre is replaced by silex ; but in this instance, as in most oth- 

 ers, these ligneous remains have the perforations of the Teredo. 



I have lately examined some Baculites from Missouri, and find 

 them specifically the same w^ith the marl fossil B. ovatus. The oc- 

 currence of this shell, together with remains closely allied to the 

 Saurocephalus, may lead us to inquire, whether there is not a forma- 

 tion in Missouri contemporaneous with that of New Jersey. 



I cannot close this paper without making a few extracts from a let- 

 ter recently addressed to me by Prof. Buckland, of Oxford, in refer- 

 ence to my papers in the Journal of the Academy. With respect to 

 the Plesiosaurus that distinguished geologist observes — " if you can 

 establish it, it will as far as our European experience goes, be deci- 

 sive in favor of secondary formations." " Your recognition of the 

 Mosasaurus is highly interesting ; we have it according to Mr. Man- 

 tell, in the chalk at Lewes; but this is a rare fossil with us; nor is it 

 much known on the European continent beyond the neighborhood of 

 Maestricht." "I entirely agree with you in thinking some of the 

 shells you mention, especially the Ammonites and Scaphites, to be 

 characteristic of secondary formations. I have never known a single 

 shell of either of these genera in tertiary beds. The Belemnite, also, 

 is almost exclusively limited to secondary : the only exception I know 

 of is the equivocal rock immediately resting on chalk at Maestricht, 

 clearly posterior tn the Chalk." 



" With regard to the mere abundance of greensand in any stra- 

 tum, we find green particles almost as abundant in tertiary beds above 

 the chalk, as in the beds immediately below it. This character, there- 

 fore, alone, will decide nothing in the question between yourself and 

 Prof. Eaton. Amber I have never seen except in tertiary beds ; 

 but I see no reason why we may not expect to find it, as we find lig- 

 nite, in secondary formations also : still, so far as our knowledge goes, 

 we must allow amber to be indicative of tertiary." 



In reference to the last remark of this eminent geologist I will 

 just observe that Cuvier and Humboldt both give instances of the oc- 

 currence of Amber with the lignite of secondary beds ; these author- 

 ities have been quoted for the fact in the former part of this synopsis. 

 Prof. Buckland's letter was written before the publication of this me- 

 moir, which I have no doubt he will admit to contain much addition- 

 al evidence in support of the position I have taken. 



With respect to the clay beds which are associated with our marls, 

 I have not hitherto been able to ascertain that they hold any uniform- 



