266 - Dr. Mac Culloch on the 
who has described them in his Conchologia Subapennina, and 
in his later essay on the soil of Rome. As he has singularly 
failed in his attempt to explain them, no less than the reviewer 
of his work in the Edinburgh Review has done, I have endea- 
voured to supply that deficiency. But in so doing I have not 
presumed to make any alterations in his facts, nor to suggest 
any other statements of them than those which he has himself 
made. On the contrary, they are taken rigidly as he has repre- 
sented them, excepting where, in a few places, the observa- 
tions appeared deficient ; and I have merely proposed to amend 
them on the principles which he has himself furnished. It 
is an extreme abuse, and, unfortunately, far’ too common 
on the part of geological writers, to determine what an ob- 
server ought to have seen. This is a practice which may be 
made subservient to any hypothesis whatever, and which 
renders all observation useless; but there is no law in philo- 
sophy against the attempt to reconcile the observations of 
others to general principles, where the observers themselves 
may have failed in this respect; provided that no liberty is 
taken with the facts that are given. 
The alluvial strata of Italy had been classed by many per- 
sons with the tertiary or fresh water formations, aud as a simpler 
race of those which have been ascertained in the vicinity of 
Paris, and in our own country. This arrangement is evidently 
improper, and it appears to have arisen from some confusion 
in the observations with respect to the marine and the terres- 
trial remains, and from having recourse to an analogy which, 
on a superficial view, seems sufficiently obvious, but which is 
inapplicable, A good deal of intricacy does in fact exist. The 
lowest alluvium is evidently of marine origin, while the upper 
one is of a terrestrial nature; and the apparent confusion, 
which has thrown error into the observations, is caused partly 
by the proximity of the two, partly by an intermixture occa~- 
sionally produced in more recent times, partly by that singular 
alluvial rock, the travertino, and in a great measure by the 
numerous volcanic alluvia, or tufas, which, instead of remain- 
