190 INDEX TO THE STRATIGEAPHY OF NORTH AMERICA. 



The limestone conglomerate referred to in the above quotation was made the 

 subject of refined study by Ruedemann/™ with reference to the faunas of matrix and 

 pebbles, origin of materials, and conditions of deposition. From this paper the 

 following summary is taken: 



1. The investigation of the lower Siluric shales of the neighborhood^ of Albany has led to 

 the observation of a conglomerate bed embedded in these shales and outcropping on Rysedorph 

 HiU near Rensselaer, on the Moordener Kill near Castleton, and at Schodack Landing. 



2. The most interesting feature of this conglomerate is the fauna which the component 

 pebbles and the matrix contain. To describe these and to obtain from them conclusive data 

 as to the age of the inclosing Normans KiU shales is the principal purpose of this paper. 



3. The conglomerate contains a great variety of pebbles. In the southern outcrops, at 

 Schodack Landing, nonfossiliferous sandstone pebbles prevail; going northward, fossiliferous 

 limestone pebbles increase, and on Rysedorph HiU they are the principal components. 



4. The limestone pebbles are shown by their faunas to be derived, in very small number, 

 from Cambric and Chazy rocks ; more frequently from the LowviUe limestone ; and prevailingly 

 from extremely fossiliferous black and gray limestone beds which are of lower or lowest 

 Trenton age. 



5. A specially interesting feature of the fauna of these Trenton pebbles was found in the 

 considerable number of new forms, largely brachiopods, trilobites, and ostracods. Some of 

 these belong to genera new to the American Trenton but weU represented by very similar forms 

 in equivalent north European beds. These, as weU as several other forms which also occur 

 in the Rysedorph Hill conglomerate and are restricted to the eastern Trenton, support the 

 conclusion derived from the distribution of the Normans Kill graptolite shales, viz, that in 

 lower Trenton time the eastern Trenton sea had attained connection with the Atlantic. 



6. As the fauna of the Trenton pebbles is in marked features different from' that of the beds 

 known in the Mohawk and Hudson valleys, it is supposed that the material was derived from 

 the regions to the east and northeast, where the Trenton beds have now become metamorphosed 

 and the fossils obliterated. 



7. The occurrence of the lower Trenton limestone pebbles in this region is taken to indicate 

 that at the beginning of the Trenton period the quiet limestone-depositing Trenton sea extended 

 also over this region; while the presence of Normans Kill shale of lower Trenton age proves 

 that this favorable condition soon came to an end, and a radical change in the physical conditions 

 took place. 



8. The conglomerate itself is intraformational. It is embedded in shale of the same age, 

 and the fauna of the matrix of the conglomerate is of lower Trenton age. The conglomerate, 

 therefore, evidently does not mark any important change in the physical condition of the 

 region but is probably due to a temporary elevation of a low Appalachian ridge into the sphere 

 of wave action. 



This conglomerate closely resembles the conglomerate of the Quebec formation 

 of Citadel Hill, Quebec, in its constitution, associated shales, and faunas. 



The full statement of Ruedemann's studies of the graptolites is contained in 

 two memoirs, to which reference should be made for adequate appreciation. The 

 first of these ^° deals chiefly with horizons discussed in Chapter II of this work; from 

 the second ^^ the following table is quoted : 



