198 PKOr. H. A. NICHOLSOK AND DE. J. MUKIE ON THE 



they resemble the other fossils in being converted into dolomite. 

 Again, in the cherty limestones of the Corniferous series (Devo- 

 nian), we find numerous Stromatoporoids in a siliceous condition, 

 and we fiad at the same time that nearly all the associated corals, 

 Brachiopods, and other originally calcareous fossils have under- 

 gone silicification. If we ascend, however, into the immediately 

 overlying shales and impure limestones of the Hamilton group, 

 we find an abundance of Stromatoporoids (sometimes specifically 

 identical with those of the Corniferous) in the same completely 

 calcareous condition as all the other fossils of the formation. 

 Similar facts seem to be observable elsewhere, as shown by the 

 fact that the Stromatoporoids of the "Wenlock Limestone of 

 Dudley, the Wenlock Limestone of Grotland, and the Devonian 

 Limestones of Devonshire resemble the fossils associated with 

 them in being calcareous*. 



The microscopic characters of thin sections afibrd comparatively 

 little help in determining whether Stromatopora was originally 

 calcareous or siliceous. In no case, however, have we succeeded 

 in observing that the laminae or pillars were composed of distinct 

 and separate siliceous granules ; nor can any structure of a similar 

 nature be detected by the microscope in siliceous examples of 

 Stromatopora from which the matrix has been completely removed 

 by the action of weathering or the use of acids. Such a con- 

 struction, however, is very readily observable in the case of Par- 

 ker ia. 



Another fact of a very noteworthy character is brought out by 

 an examination of thin sections of the Stromatoporoids with the 

 microscope. In all cases alike, namely, whether the specimen be 

 purely calcareous or partly calcareous and partly siliceous, the 

 carbonate of lime forming the actual skeleton is invariably granu- 

 lar. In other words, the skeleton is never composed of crystalline 

 calcite, but always of the granular ill-defined calcareous matter 

 which we find also in the skeleton of the ordinary corals. The 

 importance of this observation is much enhanced by the discovery 

 made by Zittel (Neues Jahrb. fiir Mineralogie, &c. 1877) that 



* Since writing the above, we have personally visited the Devonian Lime- 

 stones of Devonshire, the Upper Silurian Limestones of Wenlock Edge and 

 Woolhope, and the great " Eifelkalk " of Germany, from all of which we have 

 made large collections of Stromatoporoids ; and in all these localities we find 

 the above conclusions confirmed in all particulars. That is to say, the Stromato- 

 poroids are invariably calcareous where no silicification of the associated corals 

 and shells has taken place. 



