—. 15 = a 

- Parr IE. Sxor. ii. §7.] SUBAERIAL DENUDATION. 441 
Fyne.2 The formation of such concretions may be analogous to the 
solution and deposition of oxides of iron and manganese by organic 
acids, as on lake floors, bogs, &c. (p. 463). In connection with the 
chemical reactions indicated by these nodules as taking place on the 
sea-bottom, reference may be made to a still more remarkable dis- 
covery made by Mr. Murray in the course of his examinations of 
the materials brought up from the same abysmal deposits... He has 
detected abundant minute concretions or bundles of crystals which 
on analysis by M. Renard have been identified with the zeolite 
known as phillipsite. These crystals have certainly been formed 
directly on the sea-bottom, for they are found gathered round abysmal 
organisms. The importance of this fact in reference to the chemistry 
of marine deposits is at once obvious. 
From a comparison of the results of the dredgings made in recent 
years in all parts of the oceans, it is impossible to resist the con- 
clusion that there is nothing in the character of the deep-sea deposits 
which finds a parallel among the marine geological formations visible 
to usonland. It is only among the comparatively shallow water 
accumulations of the existing sea that we encounter analogies to the 
older formations. And thus we reach by another and a new approach 
the conclusion which on very different grounds has been arrived at, . 
viz., that the present continental ridges have existed from the 
remotest times, and that the marine strata which constitute’ so 
large a portion of their mass have been arcumulated not as deep 
water formations, but in comparatively shallow water along their 
flanks.? . | 
§ 7. DENUDATION AND Deposition.—The results of the 
action of Air and Water upon Land? 
It may be of advantage, before passing from the subject of 
the geological work of water, to consider the broad results achieved 
by the co-operation of all the forces by which the surface of the land 
is worn down. ‘These results naturally group themselves under the 
two heads of Denudation and Deposition. 
1. Subaerial Denudation—the general lowering of land. 
The true measure of denudation is to be sought in the amount of 
mineral matter removed from the surface of the land and carried 
into the sea. This is an appreciable and measurable quantity. 
There may be room for discussion as to the way in which the wasie 
is to be apportioned to the different forces that have produced it, 
but the total amount of sea-borae detritus must be accepted as a fact 
1 Nature, xviii. (1878), p. 628. 
* Proc. Roy. Geograph. Soc. July, 1879. 
* This section is mainly taken from an essay by the author, Trans. Geol. Soc. 
Glasgow, iii. p. 153. 
