66 LOWER PENINSULA. 



on the cast side to touch the shore line some miles south of Harris- 

 ville. 



In unvveathered condition the black shale is hard and slate- 

 like, of very even bedding, of finely laminated structure, fissible 

 in thin leaves, which have some degree of elastic flexibility. 

 Its color is usually a perfect black. In weathered condition, it 

 becomes grayish, and splits open into paper-thin, laminar frag- 

 ments ; exposed to the fire, it burns with a flame, but it is rarely 

 rich enough in bitumen to sustain the heat for its own combustion. 

 Seams of iron pyrites, or nodular concretions of it, and fine 

 granular crystals are invariably found pervading the beds ; also 

 subordinate seams of lime rock, which frequently have cone-in- 

 cone structure, are found interstratified ; but, most remarkable, are 

 large, spherical, calcareous concretions, which not unfrequently are 

 of almost mathematically true globe form, and sometimes of a di- 

 ameter of from 5 to 6 feet — usually, however, of from i to 2 feet. 

 The concretions must have formed while the shale mass was soft 

 and bulky ; by subsequent pressure of the incumbent sediments, 

 the shale diminished its bulk, and was bent around the hard 

 globes which did not yield to the pressure. 



The concretions are sometimes formed of a granular lime-rock 

 mass, and often inclose organic remains, as bones of fishes or 

 pieces of wood ; in other cases they have a sparry structure, with 

 elongated fibrous crystals radiating in all directions from the cen- 

 tre to the periphery. The spar crystals are usually carbonate of 

 lime darkened by bitumen, but in some localities it is a sparry car- 

 bonate of iron which composes the concretions. The analysis of 

 concretions of granular limestone structure, from the shales of 

 Norwood, gave : 



Carbonate of lime 89 per cent. 



" " magnesia 2 " 



Insoluble residue, bituminous and silicious, 7,5 " 



On the surface of some slabs of cone-in-cone structure interlami- 

 nated with the black shales, I found at Norwood large dermatic 

 plates of Aspidichthys and specimens of a Lingula : fragments 

 of wood (Dadoxylon) are likewise of common occurrence in that 

 and other localities. 



North of Norwood, the black shales form bluffs 10 or 15 feet in 



