128 
ELEMENTS OF GEOLOGY. 
deposits of mud are amassed, including the remains of Med¬ 
iterranean species. It follows, therefore, that if at some fu¬ 
ture period the bed of the Red Sea should be laid dry, the 
geologist might experience great difficulties in endeavoring 
to ascertain the relative age of these formations, which, al¬ 
though dissimilar both in organic and mineral characters, 
were of synchronous origin. 
But, on the other hand, we must not forget that the north¬ 
western shores of the Arabian Gulf, the plains of Egypt, and 
the Isthmus of Suez, are all parts of one province of terres¬ 
trial species. Small streams, therefore, occasional land-floods, 
and those winds which drift clouds of sand along the deserts, 
might carry down into the Red Sea the same shells of flu- 
viatile and land testacea which the Nile is sweeping into its 
delta, together with some remains of terrestrial plants and 
the bones of quadrupeds, whereby the groups of strata be¬ 
fore alluded to might, notwithstanding the discrepancy of 
their mineral composition and marine organic fossils, be 
shown to have belonged to the same epoch. 
Yet, while rivers may thus carry down the same fluviatile 
and terrestrial spoils into two or more seas inhabited by dif¬ 
ferent marine species, it will much more frequently happen 
that the co-existence of terrestrial species of distinct zoolog¬ 
ical and botanical provinces will be proved by the identity 
of the marine beings which inhabited the intervening space. 
Thus, for example, the land quadrupeds and shells of the val¬ 
ley of the Mississippi, of central America, and of the West 
India islands differ very considerably, yet their remains are 
all washed down by rivers flowing from these three zoolog¬ 
ical provinces into the Gulf of Mexico. 
In some parts of the globe, at the present period, the line 
of demarkation between distinct provinces of animals and 
plants is not very strongly marked, especially where the 
change is determined by temperature, as it is in seas extend¬ 
ing from the temperate to the tropical zone, or from the 
temperate to the arctic regions. Here a gradual passage 
takes place from one set of species to another. In like man¬ 
ner the geologist, in studying particular formations of re¬ 
mote periods, has sometimes been able to trace the grada¬ 
tion from one ancient province to another, by observing 
carefully the fossils of all the intermediate places. His suc¬ 
cess in thus acquiring a knowledge of the zoological or bo¬ 
tanical geography of very distant eras has been mainly 
owing to this circumstance, that the mineral character has 
no tendency to be affected by climate. A large river may 
convey yellow or red mud into some part of the ocean, where 
