On te Siton Sten Sa 91 * 
* 
"WENLOCK saat. Gos % 
Shells, - “€ Trilobite. , 
; Se ~~ Asaphus limuluras, “ 
(Leptena, Sow.) _ a (Wetherilli, Greeny 
Orthis elegantula, : ti * (longicaudatus, Murch.) 
— (canalis, Sow.) gts oe rs e ? 
hybrida, +. 3 = - ill % * 
Delthyris Vusee :* oui S eae 
CARADOC SANDSTONE. anne 
Shells. ‘ Trilobites. ah - 
Strophomena alternata, one tessellatus,* 
sericea, Trinucleu pb cca Murch, ) 
(Leptena, Blow:) Soi 9sd micropleura, 
: Orthis testadinaria, saphus micropleuru ¢ 
‘~ * callactis iCal punctata, Mure! 
, flabellulu um, . <a 
Bellerophon acutus. 
| . Observations on the Plastic Clay. 
There appears to be some difference of opinion respecting the 
relative position of this formation in the United States ; by some 
geologists it is included in the secondary or cretaceous series, and 
by others in the lower tertiary, corresponding in position to the 
_ plastic clay of England. I have examined s specime ns from both 
countries, and the only difference I can perceive, is the more 
unctuous or glossy appearance of the English specimens. If of 
the same age, it is remarkable that a tertiary deposit should extend 
‘such a distance, and preserve its mineral character and its variega- 
ted appearance so perfectly as it does, or rather that the condi- 
~~ tions for its deposition were so much the same in both countries 
Inthe same era. But this resemblance will not alone establish 
the relationship of the American with the plastic clay of Europe, 
and fossils we have none, whereby to institute a comparison. In 
land and Virginia the lower fossiliferous tertiary rests either - - 
On the primary or cretaceous rocks, and has never been found re-_ 
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* Mr. Murchison thinks his species different from the tessellatus, in 
s of having od spinous processes to the buckler; best | in this respect | there is no 
accurate observer. I cannot omit to pay a passing tribute of praise to the zeal, in- 
try talent, which have put us in possess sion of a monograph of the trilobites, 
illustrated by models, that have greatly facilitated es labors of geologists, and 
stimulated i inquiry into the history of Transition remain 
