of Burlington, loiva, and its Vicinity. 217 



lower part. In these places the crinoids are remarkably 

 well preserved, while most of the other fossils are in a bad 

 state of preservation. What little calcareous matter the 

 mass contains in these places seems to have been segre- 

 gated in such parts as contained the crinoidal remains, 

 solidifying these at the expense of the other parts. 



The stone in this part of No. 7 is, to a great extent, 

 composed of the remains of crinoids, more or less solidly 

 cemented together, and apparently constitutes the entire 

 vertical range of most of the species of these fossils found 

 in it. Above this, it becomes interstratified with bands of 

 chert, some of which reach a foot in tliickness, and are 

 quite continuous. Still farther upward, it passes into a 

 yellowish or brown silicious shale. This is also interstrat- 

 ified with bands of chert and impure limestones, forming 

 a bed of passage from No. 7 to No. 8. In some places, 

 however, this bed of passage is made up principally of the 

 cherty limestones, the silicious shale being but slightly 

 developed. 



The fossils in these chert bands are mostly in the form 

 of casts, but are of the same species as those in the calca- 

 reous strata with which they are associated. 



Although the introduction of silicious matter into the 

 waters in which these rocks were deposited did not imme- 

 diately affect the existence of its fauna, yet, from the time 

 of the commencement of its deposit in any considerable 

 amount, the species of crinoids in particular seem to have 

 commenced dying out ; and before the completion of this 

 bed of passage, nearly or quite all of the species of cri- 

 noids, together with many of the species of shells found 

 in the middle and lower portions of No. 7, ceased to exist. 

 A very few species, principally of the MoUusca, survived 

 this period, and flourished during the deposit of No. 8. 



No. 8 is generally a light gray, semi-crystalline limestone, 

 often changing to a color similar to that of No. 7, and in 



JOURNAL B. S. N. II. 28 



