No. 136.] 



49 



is well seen in many places near the Cohoes, and along the Hud- 

 son-river Railroad. In much of this disturbed region the rock 

 has been changed in texture by the forces to which it has been 

 subjected, and fossils are very rare. 



The same formation may be traced, overlying the Trenton 

 limestone, northwestward to the Mississippi (where, however, it 

 is thin and not conspicuous), northeastward to. the Lower St. 

 Lawrence, and southward along the Appalachian ranges as far as 

 Eastern Tennessee and Alabama. It is in its western extension 

 very calcareous, forming most of the great series of the " Blue 

 LIMESTONE," SO Called, near Cincinnati. 



The lower part of this formation is a fissile black slate about 

 75 feet thick, called in the State reports the 



UTICA SLATE. 



Its most marked fossil is the Tricrthrus heckii, a small trilobite, 

 which in many localities is very abundant. It is usually found 

 in fragments, and crushed flat by the settling or consolidation of 

 the deposits in which it lies buried (Fig. 9, No. 1). 



The higher strata, to which the name of the Hudson-riveh 

 GROUP is more usually restricted, are brownish slaty masses, with 

 some coarse sandstones especially toward the top, and in some 

 places, near the summit of the group, a coarse sparry limestone. 

 The fossils are in great part bivalve shells, including a number 

 of elongated forms not very unlike the " freshwater muscles " of 

 our lakes (Fig. 11). 



Some Orthocerata also occur ; and some of the same Trilobites 

 known in the Trenton limestone are found occasionally in this 

 group also, among them the Isotehcs and Trinucleus before 

 described. The coiled shell Trocholites is also found here, but 

 is not common. There are man}' indistinct remains of seaweed, 

 and a great variety of Graptolites, which arc a very peculiar 

 and as yet little understood family of animals, apparently related 

 in some way to the modern " Sea-pen," or to some of the Bryozoa, 

 They are peculiar in their appearance, usually occurring in frag- 

 ments as straight black stems about one-tenth of an inch wide, 

 having serrated edges ; but Prof. Hall has found in the northern 

 extension of this group of rocks near Quebec, more perfect speci- 

 mens showing that these stems branched or radiated from a com- 

 [Assembly, No, 136.] 7 



