198 UK. C. T. TRECHHANN ON A MASS OF ANHYDRITE [June 1913, 



found them less concentrated in the calcareous segregations, with 

 subsequent idiomorphic growths round the quartz-grains. 



Patches or beds of original unsegregated rock occur in several 

 sections of these highly-altered rocks, frequently of very limited 

 extent and irregular shape. As already stated, they are generally of 

 an intensely porous and friable nature and considerably fractured. 

 The fossils are obtained only by carefully breaking the fragments 

 along the original bedding-planes. These unaltered parts of 

 the formation afford a useful clue to the original nature and 

 palaeontological horizon of several highly-altered limestones. 1 



The above-described segregation is quite distinct from the typical 

 concretionary structure of the lower beds of the Upper Limestones. 

 Both the botryoidal bodies and their investing matrix yield a great 

 quantity of oily and carbonaceous matter on solution in acid : a 

 feature not so evident in the former segregations, and one which 

 would seem to suggest the formation of these bodies under the 

 influence of organic matter. 



Prom analogies in quantity of organic matter, nature of residues, 

 and general preservation of fossils, I am inclined to attribute the 

 ' cannon-ball ' structures to the action of segregation at a very 

 much earlier period of the formation, anterior to the great changes 

 undergone by the rock. 



The action of percolating water in leaching out the soluble con- 

 stituents originally deposited with the Magnesian Limestone has 

 already been noticed. The part which it plays in the dedolomiti- 

 zation of great tracts of the formation through the removal, in 

 great part mechanical, of the powdery magnesian constituent has 

 also been touched upon. 



The Upper Shell-Limestones are in places considerably altered 

 by the action of water, which percolates with particular ease 

 through these rocks. In Blackhall Colliery Sinking, at one time in 

 1911, as much as 15,000 gallons per minute were being pumped 

 from the Shell-Limestones. Analysis of this water showed it to 

 correspond to sea-water diluted to about five times its volume with 

 water containing calcium carbonate in solution. Previous analyses 

 had shown that the proportion of sea-water varied at different 

 levels. 



The effect is very noticeable in the so-called ' interbedded con- 

 glomerate ' of Howse, which seems to occupy a fairly definite 

 horizon in the Upper Shell-Limestones. The rounded fragments 

 are interbedded among layers of bedded limestone, and appear 

 to have formed the hard unaltered cores in a soft and friable mass 

 of limestone undergoing gradual change, the soft powdery portion 

 having been removed by water. Each mass at Blackhall Hocks 

 and elsewhere is generally surrounded by thin concentric layers 

 of altered material, which must be broken away to reach the 

 fossiliferous rock inside. 



1 It. Howse, ' Guide to the Collections of Local Fossils in the Hancock 

 Museum,' 1889, p. 10, records a fragment of a coniferous plant from a 

 laminated mass in concretionary rock near Westoe. 



