68 Cornish and Kendall—Calcareous Organisms. 
In the case of the experiment where the fossil shells were sus- 
pended in carbonated water the percentage of loss is calculated on 
the weight of the shell before its immersion in the carbonic acid 
solution in each case, and not on the original weight. It will be 
observed from the numbers given that the ratio between the losses 
by solution increases towards the end of the experiment when the 
aragonite shell had assumed a clayey consistency. 
Experiment with fossil shells suspended in CO, solution :— 
Nature deals Loss after After other After other 
of Subst, Original We. 70 brs. 48 hrs. 74 hrs. 
Calcite ...... °3196 grams, ... 12°60°/,  ... Tl Gaye k Mask 9°44 °/, 
Aragonite... °3779  ,, Soe 26330 Fie ee, LISIB ORY seen aiorelioag 
Experiment with fossil shells in a state of fine powder :— 
Nature of Subst. Original Wt. Loss in Wt. °/, of Loss on Original Wt. 
Calcite ...... O94 oramsvunnest) WitcOS29)erams..) ss.) 4eD6 
Aragonite ... 6010, di "0858, ane 14:27 
Experiment with powdered crystals :— 
Nature of Subst. Original Wt. Loss in Wt. °/, of Loss on Original Wt. 
Calcite ...... 5850 grams... "0404 grams... 6-90 
Aragonite ... SBN T 53 ae 0850  ,, au 5°65 
With regard to the question of structure, aragonite fossil shells 
have a hard surface, but the interior, though close-grained, is porous. 
The calcite shells on the other hand are compact throughout. The 
porosity of the aragonite fossils is indicated by the circumstance 
that they adhere to the tongue. The difference of structure is well 
shown by the following experiments. If a fossil calcite shell be 
immersed in water a few large bubbles collect on the shell, and after 
a time detach themselves one by one. On the other hand, if an 
aragonite fossil from which the hard outer layer has been removed 
be immersed, a stream of small bubbles rises rapidly from the shell, 
giving an appearance of effervescence similar to that produced by 
the action of a dilute acid on carbonate of lime. 
The geological conditions favourable to the removal of aragonite 
shells appear to be: 
(a) Enclosure in permeable beds. 
(6) Flow of carbonated water. 
The latter condition can of course be best complied with above 
the saturation level of the rock. This is well illustrated by the 
following observation made at the Coralline Crag pit belonging to 
Mr. Pettit, on the Leiston road, near-Aldborough, referred to in the 
paper already cited (Gnot. Mac. Nov. 1883). Here, above the 
saturation level, aragonite shells have entirely disappeared, casts 
alone remaining, whilst below the saturation level, in a shallow 
excavation made in the corner of a small pond, a bed of hard Crag 
was met with containing actual aragonite shells, though in so 
advanced a stage of decomposition that it was impossible to secure 
a specimen. It is interesting to observe that where well-marked 
lines of flow occurred, even the calcite shells were entirely removed. 
In the same connection we may quote from Mr. Sorby’s address 
