4 INTRODUCTION 



clay. This difference, however, does not appear to be due 

 directly to mineral composition, for Cornish and Kendall 

 found that when crystals of calcite and aragonite were 

 powdered and placed in carbonic acid solutions of the same 

 strength, the aragonite was not acted on more rapidly 

 than the calcite, and the same result was obtained with 

 powdered fossil shells. From all these considerations, it 

 is not surprising to find that in some strata the aragonite 

 skeletons have entirely disappeared, whereas those formed 

 of calcite remain. This will obviously be most likely to 

 occur in pervious beds through which water containing 

 carbonic acid percolates. A striking instance of the differ- 

 ence in the solubility of calcite and aragonite was furnished 

 by some specimens of the common edible mussel, Mytilus 

 edulis, in which the inner layer of the shell is formed of 

 aragonite and the outer of calcite ; Sorby found specimens 

 in the raised beach at Hope's Nose, Torquay, which had 

 lost the inner layer but not the outer. Similarly, in speci- 

 mens of Spondylus from the Chalk, the inner layer of the 

 shell has been completely removed, but the outer is left. In 

 some cases aragonite is replaced by calcite, but then the 

 organic structure is entirely destroyed, and we get merely 

 a mass of calcite crystals. Calcite is never replaced by 

 aragonite. 



The mineral character of the skeleton of the chief 

 calcareous organisms is as follows : — 



Foraminifera. — The vitreous forms consist of calcite, 

 the porcellanous probably of aragonite. 



Porifera. — Calcareous sponges of calcite. 



Anthozoa. — The Alcyonaria are of calcite, except He- 

 liopora, which is of aragonite ; the Madreporaria are of 

 aragonite. 



