162 
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 flows, 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 
