1872.] 



e Shearwater 3 Scientific Researches. 



565 



" Geologist of the future " would scarcely thiuk himself justified in limiting 

 the " Cretaceous Formation " to beds distinguished only by the presence 

 of a certain set of Animal forms not to be found in those above them. If, 

 however, he should think proper so to limit that term, he would have to 

 invent a new one for the later beds deposited in absolute continuity with it, 

 and perhaps in all but one particular under the same conditions. That 

 particular, if Mr. Prestwich's very ingenious speculation be accepted as 

 provisionally true, was Temperature ; a reduction in that of the Cretaceous 

 Sea, consequent upon the opening of a communication with the Arctic basin 

 (by changes in level perhaps thousands of miles off), being an adequate vera 

 causa for the disappearance of all those animals which are dependent on a 

 warmth approaching the Tropical ; whilst those which could adapt them- 

 selves to the change (perhaps with some modification of structure) would 

 maintain their ground, and would in their turn leave their remains to be 

 entombed in the mass of Globigerma-mud, the production of which on the 

 Atlantic sea-bed seems to have gone on uninterruptedly through the whole 

 of the Tertiary Period to the present time. 



45. I have thought it not inappropriate to conclude this portion of my 

 present Report with a recurrence to a subject brought into prominence by 

 Prof. Wyville Thomson and myself three years ago ; partly because I thus 

 obtain an opportunity of explaining what was really meant by a somewhat 

 incautious expression, and partly because a new value is given to the doctrine 

 which that expression was intended to convey by Mr. Prestwich's explicit 

 acceptance of it, coupled with his ingenious application of the doctrine of 

 a General Oceanic Circulation to account for the disappearance of the more 

 Tropical portion of the Cretaceous Fauna, without change in any other 

 condition than Sea Temperature. — This, as it appears to me, is a singularly 

 felicitous example of the valuable results which may be expected from the 

 wider extension and more systematic prosecution of the inquiries in which 

 it has been my privilege to take part, and which I now leave in the hands 

 of the valued Colleague by whom they were originally suggested ; enter- 

 taining the fullest confidence that they will be carried on with zeal and 

 ability, and a sanguine hope of their distinguished success. 



Part II. 



Further Investigation of the Gibraltar Currents. 



46. The inquiries made in the 'Porcupine' Expedition of 1870 having 

 shown that the information obtainable by the Hydrometer respecting the 

 stratification of Atlantic and Mediterranean water in the Strait of Gibraltar 

 might afford valuable evidence in regard to their movement, I made the 

 determination of Specific Gravities my own special charge, whilst Capt. 

 Nares, in consultation with me, carried out the Mechanical portion of the 

 investigation. As his separate Report of the inquiries he conducted, and 

 of the deductions he drew from them, has been already communicated to 



