33 



The Lower Shales, or the Hamilton Shales Proper. Only 

 about a foot of these is exposed at the base of Section 5, but 

 this foot of shale contains a large number of interesting 

 fossils. Immediately below the Encrinal limestone the shale 

 is practically barren for a thickness of three or four inches. 

 Even calcareous matter seems to be absent from it, and the 

 shale is soft, light colored and easily cut with a knife. If it 

 is exposed to the atmosphere and the heat of the sun, it 

 hardens, by the evaporation of the water which it 

 contains, but on soaking, it becomes a tenacious mud. 

 This character is due to the leaching out of the calcareous 

 matter by the waters which carried sulphuric acid, derived 

 from the oxidation of the iron sulphide on the under side of 

 the Encrinal limestone. Below this decalcified mass of shale 

 is a bed an inch or less in thickness, which is made up mainly 

 of three classes of fossils, viz : A small, flat, branching 

 bryozoan, Stictopora incisurata Hall, a small brachio-' 

 pod with matted spines all over its exterior, Nuckospira 

 concinna Hall and a large number of the joints of crinoid 

 stems. These three forms occur in such numbers, and they 

 are usuallv so firmlv cemented, that the bed becomes a solid 

 limestone. Where it has been exposed for a considerable 

 length of time, the fossils have weathered out completely, so 

 that they may be picked up in a perfect state of preservation. 

 This bed has been called the Stictopora bed. It is the high- 

 est true Hamilton bed which has a distinct association of 

 fossils. Throughout it, and in almost every bed below, the 

 t\^pical Hamilton brachiopod Spirifer mucronatus (Conrad) 

 occurs. This is frequently furnished with long mucronate 

 points or lateral extensions, and in the Stictopora bed it is 

 represented mainly by the separated valves. The species is 

 practically restricted to the Hamilton shales*, where it is 

 abundant, only a few fragmentary specimens having been 

 obtained from the higher beds. It therefore constitutes the 

 index species of this lower fauna — the Spirifer mucronatus 



•It occurs however, iu the transition shales of the Marcellus. 



