No. 136.] 69 



estimated by Prof. Rogers at seventeen hundred feet. The group 



extends westward through Ohio (where it, with the succeeding 



Chemung group, forms what are called the Waverly sandstones) ; 



but while in Indiana it and the Chemung group together are not 



more than 400 feet, where it reaches the Mississippi and passes 



into Iowa, it and its associated rocks have so thinned out, that 



the whole vast series from the Hamilton to the Chemung groups i 



(inclusive of both) are reduced from the 3000 feet of thickness 



which they have in Middle and Eastern New- York, to less than 



200 feet. 



It is generally very poor in fossils ; so that in some places a 

 whole day, spent in searching for them, is rewarded with but a 

 few obscurely preserved shells or even none. Those described in 

 the Report on the Fourth District are shown in Fig. 37, 38 and 

 39. The crinoid in Fig. 39 is one of the most perfect and beau- 

 tiful which are known from the rocks of any age, and is a good 

 example to show the general form and structure of this curious 

 and interesting family. It has been found in perfection only in 

 the shore bluffs of Lake Erie, in the town of Portland, Chau- 

 tauque county. 



The soft slates of the Portage group contain many of the con- 

 cretions known as Septaria, described in our notice of the Mar- 

 cellus shales. 



To the Portage group succeeds the 



CHEMUNG GROUP, 



so called from being well exhibited at the " Narrows " of the 

 Chemung river. Its thickness of 1000 or 1500 feet is made up 

 of a series of thin-bedded sandstones with intervening shales, 

 and occasionally beds of impure limestone mainly formed by the 

 materials of fossil shells. It, in many places, abounds with fos- 

 sils. Trilobites are very rare, as they are in all strata above the 

 Tully limestone ; and the few found in this group (apparently 

 the same with Phacops hufo of the Hamilton group) are the last 

 of their race known in America, the whole family having been 

 extinct since before the commencement of the Carboniferous 

 period. Plants are not uncommon, and some specimens of fern- 

 like character plainly indicate a gradual approach to the forms 

 so abundant in the Coal formation. The shells are chiefly 

 bivalves, of which several illustrations are here given (Fig. 40, 

 41). It will be seen that they resemble in considerable degree 



