OR UPPER NEW-YORK ROCKS. 199 



Summary of facts respecting the upper rocks of the new-york, silurian, and 



devonian sy'stems. 



1. The series of rocks above the Tully limestone consists of alternating masses of sand- 



stone, slate and shale. The greatest mass of slate is the Genesee slate ; and the 

 greatest mass of sandstone, in continuous beds, is the Portage group. 



2. The rocks, from the Genesee slate to the conglomerates of the Coal, form one series ; 



and though this series is divided into groups, the subordinate divisions are made for 

 conxenieiice rather than utility or necessity : they serve, however, one or two pur- 

 poses, namely, those parts of the series which have intercalated members, or other 

 differences, are more fully brought to view, the economical portions may be clearly 

 defined, and the comparison of two distant points is made more striking. It will be 

 said that the groups are important, and an appeal may be made to the fossils for 

 sustaining the position. The better division of the series seems to be into upper 

 and lower, or upper, middle and lower. The division of the rocks above the 

 Taconic, and below the Coal, into two great sjstcms, the Silurian and Devonian, 

 simplifies the study of the geology, and encumbers the mind of the student less than 

 that which makes many subordinate parts. The deepest part of the Devonian sea 

 appears to have been in the region of the Catskill series ; and if we may form an 

 opinion of the continuous depth of such a sea, from the extension and thickness of 

 a formation, it would seem that the depth increased rapidly upon the eastern shore, 

 but shallowed more slowly to the southwest. This view seems to be sustained by 

 the fact that the prolongation of the Silurian and Devonian systems eastward is 

 quite limited, some of the beds of the Lower Silurian extending only five or six 

 miles east of the city of Hudson ; while in order to place ourselves in the midst of 

 a deep silurian and devonian sea, we have only to travel ten miles southwesterly 

 from this city. The whole mass composing both systems disappears at once, as it 

 were, on the eastern side, thinning out suddenly ; and the Taconic slates, plunging 

 down at a steep angle, form a basis upon which the whole is supported. 



3. There is less difference between the lower part of the Devonian and upper Silurian in 



New-York, than there is between the Champlain and the Ontario divisions. 



4. The economical products are fine and valuable Hags, quarries of which may be opened 



through a wide horizontal as well as vertical range. The rock contains neither 

 ores, limestones, nor brine springs. 



5. Some of the springs, which issue from the Genesee slate, are hydrosulpliuretted in an 



eminent degree ; while the springs of the rocks above the slate, are pure as those 

 of a primary district. 



6. The country underlaid by these rocks is hilly, and the slopes afford an excellent soil 



for grazing. Wheat, though not the natural crop, is still raised on the bottoms of 

 the narrow valleys. 



