- TESTS OF THE AGES OF ROCKS. 
125 
globe had been environed by a succession of distinct aque¬ 
ous formations, disposed round the nucleus of the planet, 
like the concentric coats of an onion. But, although, in fact, 
some formations may be continuous over districts as large 
as half of Europe, or even more, yet most of them either ter¬ 
minate wholly within narrower limits, or soon change their 
lithological character. Sometimes they thin out gradually, 
as if the supply of sediment had failed in that direction, or 
they come abruptly to an end, as if we had arrived at the 
borders of the ancient sea or lake which served as their re¬ 
ceptacle. It no less frequently happens that they vary in 
mineral aspect and composition, as we pursue them horizon¬ 
tally. For example, we trace a limestone for a hundred 
miles, until it becomes more arenaceous, and finally passes 
into sand, or sandstone. We may then follow this sand¬ 
stone, already proved by its continuity to be of the same 
age, throughout another district a hundred miles or more in 
length. 
Organic Remains. —This character must be used as a cri¬ 
terion of the age of a formation, or of the contemporaneous 
origin of two deposits in distant places, under very much 
the same restrictions as the test of mineral composition. 
First, the same fossils may be traced over wide regions, if 
we examine strata in the direction of their planes, although 
by no means for indefinite distances. Secondly, while the 
same fossils prevail in a particular set of strata for hundreds 
of miles in a horizontal direction, we seldom meet with the 
same remains for many fathoms, and very rarely for several 
hundred yards, in a vertical line, or a line transverse to the 
strata. This fact has now been verified in almost all parts 
of the globe, and has led to a conviction that at successive 
periods of the past, the same area of land and water has 
been inhabited by species of animals and plants even more 
distinct than those which now people the antipodes, or 
which now co-exist in the arctic, temperate, and tropical 
zones. It appears that from the remotest periods there has 
been ever a coming in of new organic forms, and an extinc¬ 
tion of those which pre-existed on the earth; some species 
having endured for a longer, others for a shorter, time; while 
none have ever reappeared after once dying out. The law 
which has governed the succession of species, whether we 
adopt or reject the theory of transmutation, seems to be ex¬ 
pressed in the verse of the poet— 
Katura il fece, e poi ruppe la stampa. Ariosto. 
Nature made him, and then broke the die. 
