1G2 OLDER PLIOCENE PERIOD. [Ch. XII. 



to the geologist, not to infer too hastily a contemporaneous 

 origin from identity of mineral composition. The deposit of 

 the Upper Val d'Arno occurs nearly at the bottom of a deep 

 narrow valley, which is surrounded by precipitous rocks of 

 secondary sandstone and shale (the macigno of the Italians and 

 greywacke of the Germans). Hills of yellow sand, of con- 

 siderable thickness, appear around the margin of the small 

 basin, while, towards the central parts, where there has been 

 considerable denudation, and where the Arno ilows, blue clay 

 is seen underlying the yellow sand. The shells are of fresh- 

 water origin, but we shall speak more particularly of them 

 when we discuss the probable age of this formation in the 

 sixteenth chapter. We desire, at present, to call the reader's 

 attention to the fact, that we have here, in an isolated basin, 

 such a formation as would result from the waste of the conti- 

 guous secondary rocks of the Apennines, fragments of which 

 rocks are found in the sand and conglomerate. We should ex- 

 pect that if the freshwater beds were removed, and the barrier 

 of the lake-basin closed up again, similar sediment would be 

 again deposited, for the aqueous agents would operate in the 

 same manner, at whatever period they might be in activity. 

 Now, the only difference, in mineral composition, between the 

 lacustrine deposit above alluded to, and the ordinary marine 

 strata of the Subapennine beds, consists in the absence of cal- 

 careous matter from the clay, the torrents flowing into the lake 

 having passed over no limestone rocks. 



The lithological character of the Subapennine beds varies in 

 different parts of the peninsula both in colour and degree of soli- 

 dity. The presence, also, or absence of lignite and gypsum, and 

 the association or non-association of volcanic rocks, are causes 

 of great local discrepancy. The superposition of the sand and 

 conglomerate to the marl, on the other hand, is a general point 

 of agreement, although there are exceptions to the rule, as at 

 San Quirico before mentioned. The cause of this arrangement 

 may be, as we before hinted, that the arenaceous groups were 

 first formed on the coast where rivers entered, and when 



