TRANSACTIONS OF SECTION C. 791 
_ Similar tests were applied to the Pteropods Cavolinia and Stiliola. No com- 
pletely satisfactory optical figure could be obtained, though the optical test seemed 
to indicate a biaxial substance (Aragonite) ; but the sp. gr. determinations many 
times repeated were conclusive that those Pteropod-genera are Aragonite. 
Conclusions :—1. The effect of difference of thickness of calcareous shells upon 
their rate of solution is quite insignificant in comparison with that of difference of 
mineral constitution; thus in the Coralline Crag shells of Voluta and Cyprina 
(Aragonite), a third of an inch thick, have been quite removed, while the delicate 
Polyzoa (Calcite) which encrusted them are perfectly preserved, together with 
remains of Vitreous Foraminifera (Calcite). 
2. There is no noteworthy difference in thickness between the Pteropods and 
Globigerine. 
3. Pteropod-shells consist of Aragonite, while Globigerina and all other 
Vitreous Foraminifera examined are composed of Calcite. 
4. The disappearance of Pteropods at 1,500 fathoms, while the Globigerine 
extend to 2,925 fathoms, is due to the mineral character of the shells, and not to 
their thickness. 
8. On the Conditions under which the Upper Chalk was deposited. 
By Percy F. Kenpatn, 7.G.8, 
Attempts to determine the approximate depth of the Chalk sea from the 
comparison of the Cretaceous fauna with the Molluscan inhabitants of the existing 
seas are unsatisfactory, because there are no grounds for the belief that the low 
temperatures at present found in the ocean depths prevailed in Cretaceous times; 
hence temperature did not limit distribution to the extent that it does now. 
Solution dependent upon the depth of water would, however, act as it does in 
existing seas, and the author has applied certain principles stated in another paper 
read before the Section to the case of the Upper Chalk. 
Calcareous organisms consist in some cases of aragonite, and in others of 
calcite. Aragonite in organic structures is so much more soluble than calcite 
(though of identical chemical composition) that gigantic aragonite shells may be 
‘completely dissolved, while calcite Foraminifera exposed to exactly the same 
conditions remain perfectly preserved. 
The distribution in depth of the Pteropod Ooze of the tropical seas indicates the 
depth at which slender aragonite shells are diesolved. Pteropods swarm in the 
surface waters in such numbers that the sea is literally thick with them, yet, being 
composed of aragonite, their remains practically disappear from the oozes in depths 
exceeding 1,500 fathoms, and only sporadic examples are met with. The remains 
of globigerin», which live side by side with the pteropods, survive by virtue of 
their calcite composition down to 2,925 fathoms, nearly twice the depth, These 
facts seem to show that 1,500 fathoms is the depth at which the more delicate 
aragonite shells yield to solution. 
Turning to the Upper Chalk, we find that all aragonite structures, large and 
small, have been wholly dissolved away, while calcite Foraminifera and Polyzoa 
are well preserved and retain their fine markings. 
The question arises, When did the solution take place? ‘To this we may answer 
with some confidence that it has been effected mainly prior to consolidation, for 
chalk is a rock which takes and preserves impressions remarkably well; yet casts 
of aragonite shells are extremely rare, and are almost invariably of large and 
robust shells. The Cephalopods furnish the best illustrations of these facts; the 
phragmoccne of Belemnitella mucronata, an aragonite structure, has never been 
found in this country, though the guards (calcite) occur by thousands. If the 
solution of the phragmocones had taken place subsequently to deposition, empty 
alveoli would be found ; but in no case has the author seen a Belemnitella in this 
condition, but always with the alveolus filled with chalk, 
Casts of Ammonites (aragonite) are very rare in the Upper Chalk, such as occur 
being usually of very large size, but the Aptychi (calcite) of small species are 
occasionally found well preserved. Many considerations render it probable that 
