1848.] MURCHISON ON THE STRUCTURE OF THE ALPS. 28/ 



Columbella ihiara, Bon., M. Nerita proteus, Bon., M. 



turgidula, Bell., M. Natica lielicina, Br., P.M. 

 Mitra scrobiculata, Br., M. pseudo-epiglottina, E. Sisrnd., M. 



pyrainidella, Br. millepunctata, L., P.M. 



Conus antediluvianus, Brug., M. Ringicula Bonelli, Desk., M. 



bisulcatus, Bell, et Mich., P. buccinea, Desk., M. 



Brocchii, Bronn, P. striata, E. Sismd., M. 



Chenopus pes-graculi, Phil., M. Turritella subangulata, Br., M. 

 Turbo fimbriatus, Bronn, M. varicosa, Br. 



Solarium monilif crura, Bronn. Bulla uniplicata, Bell., M. 

 Phorus testigerus, Bronn, M. 



As the ascending series, in which intermixture takes place, is of 

 considerable dimensions, and as even close to Chieri we still meet with 

 a great number of Superga species, it is evident that a considerable 

 thickness of beds may be classed either as miocene or pliocene, 

 according to the forms which the observer may happen to meet with. 

 Amidst the species collected from these blue marls, which are geolo- 

 gically subapennine (Castelnuovo and Pino), those marked M. exist 

 in the miocene of the Superga ; those marked P. are exclusively plio- 

 cene ; and the individuals with the affix P.M. are common to the 

 Superga and the true pliocene. Out of the 95 species, then, found 

 in this one zone of blue marl, 16 are peculiar to it, 52 are known in 

 the miocene, 10 in the pliocene, and 17 are common to the two for- 

 mations. 



The citation of this important fact teaches us, that the more closely 

 the artificial limits of what geologists call formations are worked out, 

 the more impossible will it be to draw fixed lines between natural 

 groups of strata which, like these, have succeeded to each other 

 without physical disturbances. At all events, wherever the different 

 members of the same system so graduate into each other strati- 

 graphically, mineralogically, and zoologically, the tints of colour by 

 which they are characterized in a map should also be blended along 

 such mixed frontiers. 



In passing from the sandy beds in question by Castel Montalto to 

 Pino and Chieri (the angle of inclination diminishing as we recede 

 from the higher ground), the masses of which we have been speaking 

 are conformably overlaid by a great thickness of yellow sands with 

 some inosculating marls, which constitute the true subapennine beds 

 of the Astesan, so well known to geologists through the works of 

 Brocchi and others. In these uppermost beds nearly all traces of 

 anything purely miocene have disappeared, and we are immersed in 

 that same type of shells with Panopcea Faujasi, &c. which at St. 

 Gallen and other places characterizes the marine molasse of Switzer- 

 land (see ante, p. 230) *. I specially, then, caution geologists against 

 employing that term in a sense which is to convey an idea of age, for 

 as used at Turin the word molasse is exclusively applied to the strata 

 of true miocene age, whilst in Switzerland the greater part of it is 

 pliocene. Again, the pliocene deposits in Switzerland are hard sand- 



* This miocene is laid down in Collegno's map, but in my opinion, as above 

 explained, a great error prevails in that part of it which represents the eastern 

 portion of the Monferrato as cretaceous. 



X 2 



