Ixviii PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



per sandstones alternate with the lower San Casciano beds, according 

 to Prof. Merian. Ammonites occur in the Hallstadt limestone, 

 sometimes called Upper Muschelkalk, and considered the equivalent 

 of the San Casciano beds, thereby connecting these truly Triassic beds 

 with those Liassic beds above the Dachstein, in which numerous 

 species of Ammonites abound. In the intermediate dolomites no 

 fossils have yet been found. Nor have Ammonites yet been discovered 

 in the Dachstein limestone, yet they abound in the overlying Kossen 

 strata, which are considered to be the equivalents of the lower Lias 

 by M. Siiss, and to be upper San Casciano by Prof. Merian and his 

 friend M. Escher. Thus, wherever we find the strata conformable, 

 we have a confirmation of the well-known saying, "Natura non facit 

 saltum." In fact, all natural changes are gradual under these cir- 

 cumstances. The conditions of life gradually change, and the organic 

 forms are modified to meet these changes ; certain species disappear, 

 while others, adapted to the altered circumstances, are called into 

 existence, and continue to flourish side by side with some of the pre- 

 existing forms ; thus confirming the view already stated, that where 

 the strata are conformable, no line can be drawn between successive 

 formations, — the gradual change is not marked by sudden breaks 

 in the series of animal life. In fact, we must not forget that our no- 

 menclatures are for the most part only relative. Nature ever acts on 

 one long unbroken plan, and knows as little of sharp limits between 

 Trias, Lias and Jurassic, as between the families and genera of existing 

 organic life. These terms are at best but temporary shifts to assist our 

 memories, and to enable us to register our facts and our knowledge ; 

 and we must be careful not to give too much importance to nomen- 

 clatures which deserve at the best but a secondary consideration. I 

 may have occasion to allude to this question again when referring to 

 the progress of tertiary geology. I will only here observe, that I 

 think M. Merian has exercised a wise judgment in making the San 

 Casciano beds for the present a separate group, intermediate between 

 the Trias and the Lias. 



I cannot conclude these remarks without alluding to what I am 

 sure every British geologist will consider an oversight on the part of 

 the two Austrian geologists I have mentioned, in having omitted all 

 allusion to the exertions on two separate occasions of Sir R. Murchi- 

 son, and to the credit he deserves for having been the first to point 

 out the true relations of these great Alpine formations. In the Geo- 

 logical Map of the Eastern Alps, the first ever constructed, and pub- 

 lished in the third volume of our Transactions (2 Ser.), Sir R. Mur- 

 chison and Prof. Sedgwick clearly laid down on the northern flank 

 of the Alps a distinct series of rocks between the older slaty rocks 

 (Verrucano) and the Liassic and Oolitic groups. To these they 

 gave the name of Keuper, Muschelkalk, Bunter Sandstein, and 

 Rauchwacke. This zone is also laid down on their map on the south 

 flank of the Alps, passing through San Casciano, thereby showing 

 that at that early period they had recognized the identity of the beds 

 on the north flank with those of San Casciano and its vicinity, 

 although San Casciano itself was not then alluded to. Again, in 



