188 ME. E. WETHEEED Olf INSOLUBLE EESIDUES FEOM 



and shell-fragments occur: but these do not contribute to the 

 structure of the rock to any great extent. 



(ii.) Blaclc Bock. — These limestones have been made famous 

 from the fact that it was from these beds that Agassiz obtained 

 some of the fish-remains T\^hich he figured in his book on fossil 

 fishes ; and some of the originals are now preserved in the Bristol 

 Museum. Fish-remains are still to be found in the " Black Rock," 

 especially at the base. Under a microscope thin sections of the 

 limestone exhibit the spines of Productus and Sjnrifera, shell- 

 fragments, valves of Ostracoda, and Polyzoa ; but by far the greater 

 portion of the rock-structure consists of the joints of Encrinites. 



Middle Limestones. 



Passing from the Lower Limestones, we come to the base of the 

 Middle Series, which is represented by the oolitic limestone before 

 referred to. In the majority of instances the nuclei of the granules, 

 when recognizable, are Poraminifera ; but in one thin section of the 

 rock a joint of an Encrinite was discovered as the centre in two 

 granules. 



The beds which follow the oolitic limestone appear to rest 

 unconformably on these below ; but this is not real, and is due to 

 false-bedding in the oolitic limestone. The succeeding stratum is, 

 however, of a very different character from that upon which it rests, 

 and is defined by a sharp demarcation as though the alteration 

 of conditions had been rapid and without gradual passage. Thin 

 sections show the rock to contain a few Poraminifera, valves of 

 Ostracoda, and a number of circular objects measuring from -003 to 

 •012 of an inch in diameter. These calcareous bodies, whatever 

 they may be, are very numerous, and exist throughout the Middle 

 Limestones above the oolitic beds. But the most important form 

 of life, as distinguishing these beds from others, is the occurrence 

 of the genus Mitcheldeania, which, so far, appears to be limited to 

 this horizon. Following the Mitclieldeania-LimeQionQS,, as I propose 

 to call them, we come to the most important development of the 

 Carboniferous Limestone at Clifton, which extends as a bold line of 

 cliffs for a distance of nearly a mile along the Grloucestershire side 

 of the gorge of the Avon, and on the Somersetshire side appears as 

 wooded slopes. It would not be correct to say that the whole of 

 this great thickness of limestone is built up of Poraminiferous ooze, 

 but that certainly may be said of some portion. Taking it as a 

 whole, the rock is made up of Poraminifera, the small circular bodies 

 referred to in the MitcliddeoMia-JAmQ^ioiie^., occasional valves of 

 Ostracoda, shell-fragments, and some other organisms which have 

 not yet been described. 



'U])j)er Limestones. 



Towards the top of the Middle Limestones the strata become 

 more arenaceous, until beds are met with which contain as much 

 as 81 per cent, of siliceous residue ; with these true limestones occur. 



