122 SUMMER IN A BOG. 



' ' In composition some variety is observable. Very- 

 many of the smaller ones consist of the crystallized 

 bi-sulphide of iron, and some of them consist of a sym- 

 metrical shell or scale of this substance around a 

 softer nucleus, which gives them a strong resemblance 

 to an iron casting. The nodules of this kind invari- 

 ably contain lime and iron, the former substances 

 being sometimes found at the center in the shape of 

 calc spar, and more frequently occurring in a dark 

 semi-crystalline mineral that is quite characteristic. 

 The iron is always intimately associated with the lime 

 and gives to the weathered concretions the dark yellow 

 or ochreus color that marks them all in this stage. 



"Rarer substances are sometimes met in the crys- 

 talline nuclei of the concretions. Heavy spar is one 

 of these. As is now well known, there are sometimes 

 found in the centers of these bodies organic nuclei, 

 and among these are some of the most interesting and 

 remarkable fossils of the entire geological series. 

 Wood is not uncommon, a species of ancient pine, in 

 particular, the dadoxylon Newberri." 



The great fisk bones, however, are the most 

 remarkable forms to be met with here, and fur- 

 nish a link in geological discovery of unique 

 value. 



The northern part of Franklin County, 

 Ohio, is rich in the Huron shale from which 

 these concretions are taken. They are gener- 

 ally partially decomposed when found, espe- 



