xxx PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 
D’Archiac to the position in the front rank of European geologists, 
which he occupied at the time of his death, commenced, by the pub- 
lication of a ‘Résumé dun Mémoire sur le terrain tertiare du dé- 
partement de l’Aisne.’ This inaugural essay was followed, in 1838 
and 1839, by others upon the tertiary and cretaceous deposits of 
France, Belgium, and England. Of these, the ‘ Observations sur le 
groupe moyen de la Formation Crétacée’ requires particular notice, 
as a piece of thorough paleontological and geological work, under- 
taken not for the mere purpose of determining the geology of 
the district, but with the more important object of ascertaining 
whether a properly chosen district might not be made to contribute 
to the solution of a problem in general geology. This problem is 
thus formulated by d’Archiac, ‘‘ How are the fossils of a formation 
distributed in the different stages of that formation? And what 
modifications or changes do the species undergo, on the one hand, 
in time, as we pass from one stage to another, and, on the other, 
in space, as we examine the formation at different points of its geo- 
graphical extent ?” 
In order to solve the problem thus stated, M. d’Archiac observes 
that it is necessary to select a formation which can be studied upon 
its circumference, and at a great number of intermediate points, 
which has not undergone any serious dislocation, and all the stages 
of which present definite marks by which they can be compared. 
The Cretaceous formation, stretching from Burgundy to Dorset- 
shire, appearing to fulfil these conditions, was therefore subjected 
to a minute and exhaustive study, and it yielded the following re- 
plies to the questions proposed :—The more the different stages of a 
formation are developed, the more distinct are the organisms which 
they contain, or, in other words, the smaller is the number of species 
common to any two of them. Further, as the number of the mem- 
bers of the same formation diminishes, on the one hand, the species 
of the different stages tend to become mixed together; and, on the 
other, new species, and even new genera, appear in inverse propor- 
tion to the number of the stages which persist. Thus the fossils at 
the margins of the Middle Cretaceous formation differ from those of 
its centre, and, moreover, they differ geographically. The cretaceous 
organigms inhabit three zones, a northern, a middle, and a southern, 
and these have a general direction from N.W. to 8.E., which pro- 
bably corresponds with that of the isothermal lines of the period. 
The sixth volume of the ‘ Transactions’ of our Society is adorned 
by a very elaborate memoir “On the Fossils of the older Deposits 
in the Rhenish Provinces,” in which, in conjunction with our dis- 
tinguished foreign member M. de Verneuil, M. d’Archiac subjects 
a great section of paleeozoic life to a similar investigation, and con- 
clusions of no less importance are there stated :—* If the develop- 
ment of paleozoic organisms be considered relatively to the thick- 
ness of the beds, or the duration of the epoch, we shall see, Ist, that 
the total number of species always increases from below upwards ; 
2nd, that the progression is very different im each order and in each 
fomily, and that this progression is even frequently imverse, either in 
