126 Dr oh the Modern Strata. 
imbedded in a brown clay. The species are similar to thoSe 
noticed by Captain Laskey, and still common in the estuary of 
the Frith of Clyde. They are considered as occurring about 
22 feet above the present level of the sea. 
To these examples of marine diluvium others might be add- 
ed, which have been observed in England, as in Essex, &c. — 
Geol. Trans, i. p. 330. And it is probable, that many more re- 
main to be investigated. The shells which occur, being those 
of the present seas around the British Isles, furnish the means 
of distinguishing the modern marine diluvium from depositions 
of a similar character, which have taken place during earlier 
epochs, and in which the materials are yet in an unconsolidated 
state ; as in the different formations above the chalk. The ma- 
rine shells, however, in these last, do not belong to the present 
race of animals. 
6. Volcanic Deposits. — These, whether of lava, ashes, or en- 
crustations round the margin of hot-springs, may cover all the 
strata already enumerated, or be covered by them. Fortunate- 
ly for this country, such deposits, with a single exception, are 
absent. On the 20th October 1755, a shower of black dust fell 
in Zetland. It resembled lamp-black, but smelled strongly of 
sulphur. — Phil. Trans, iv. p. 297. A similar dust fell on a 
ship belonging to Leith, on the 23d or 24th of October, when 
between Zetland and Iceland, and about 25 leagues distant from 
the former. — Phil. Trans, xlix. p. 510. 
In viewing these different groups of modern strata, it is sur- 
prising to observe the various causes which may have been con- 
cerned in their production, and the intermixture of the indivi- 
duals of the animal and vegetable kingdom, of fresh-water and 
terrestrial production, with those of the ocean. A most interest- 
ing question here presents itself, and one which involves some of 
the most important speculations in geology. Are we to consi- 
der the causes by which the different modern strata have been 
produced, as analogous to those which have contributed to the 
formation of the strata, belonging to the more ancient epochs of 
the Earth’s history ^ If this question be answered in the affirma- 
tive, the occurrence of fresh and salt water deposits in the same 
hollow, and the remains of land and sea animals in the same bed, 
will cease to excite our surprise ; and many of the irregularities, 
