Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/52

xxx D'Archiac to the position in the front rank of European geologists, which he occupied at the time of his death, commenced, by the publication of a 'Résumé d'un Mémoire sur le terrain tertiare du département 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 palæontological and geological work, undertaken 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 geographical 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 Dorsetshire, appearing to fulfil these conditions, was therefore subjected to a minute and exhaustive study, and it yielded the following replies 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 members 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 proportion 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 organisms inhabit three zones, a northern, a middle, and a southern, and these have a general direction from N.W. to S.E., which probably 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 distinguished foreign member M. de Verneuil, M. dArchiac subjects a great section of palæozoic life to a similar investigation, and conclusions of no less importance are there stated:—"If the development of palæozoic organisms be considered relatively to the thickness of the beds, or the duration of the epoch, we shall see, 1st, that the total number of species always increases from below upwards; 2nd, that the progression is very different in each order and in each family, and that this progression is even frequently inverse, either in