No. 136.] 63 



fissile, short-fractured slate, one or two hundred feet in thick- 

 ness, in most places containing layers of impure limestone and 

 rounded concretions of similar material in its lower part.* These 

 slates closely resemble those of the Coal formation, and some- 

 times contain thin seams of coaly or bituminous matter, which 

 have misled many persons to spend considerable sums in digging 

 and boring in them, with the ill-founded expectation of finding- 

 useful layers of coal : an idle hope, for they lie (as will be seen 

 hereafter) thousands of feet below that true Carboniferous 

 system, beneath which no valuable coal strata have ever been 

 found. 



Their fossils are usually few, those most common in the slate 

 itself being figured in the annexed wood cut (Fig. 28). They all 

 belong to genera which have been heretofore noticed, except 

 the DisciNA and Avicula. The former was a flat circular brachio- 

 podous shell, the valves not articulated by a hinge, and with a 

 slit or hole in the lower valve through which passed a "pedicle " 

 or muscular appendage by which the animal attached itself to 

 stones or other objects on the sea bottom. The Avicula are 

 bivalve shells having the lower valve much smaller than, and often 

 quite unlike the upper, and with a wing or prolongation of the 

 hinge-line on one or both sides. They are very numerous as 

 fossils in the older rocks, but comparatively few are known now 

 living. The "Pearl 03^ster '' is one of this family. 



But the limestone layers near the base of the slate contain a 

 very peculiar group of fossils, met with in no other stratum. f 

 They are chiefly large chambered shells, Orthocerata and Gonia- 

 tites, though the latter are much like Nautilus in general form. 

 The larger species is sometimes a foot in diameter, its outer sur- 

 face beautifully marked with waving lines of growth ; and broken 

 specimens show perfectly the sinuous form of the partitions 

 between the chambers, and the siphuncle or connecting tube 



* These concretions (found also in other slaty formations) are generally cut in various 

 directions by veins of spar, the edges of which show on the surface in intersecting lines. 

 The common oval form of the concretions, and a frequent approach to regularity in the veins, 

 give them a curious resemblance to large tortoises; though they are simply concretions with- 

 out any fossil character, except that fossils are sometimes contained in them. Such masses 

 are called by the general name of Septaria. 



t This should be understood of the layers east of Marcellus. At Le Roy and elsewhere in 

 Western New -York, there are one or two thin layers of compact limestone, containing a few 

 Brachiopoda and Trilobitee, the latter appearing to be like the smaller forms of the Hamilton 

 tjroup. These laj'ers are different from those above described. 



