155 
The structural changes which characterise the surface of 
the inorganic world are of aqueous and igneous origin. The 
first of these forces is still visible, though in a less degree in 
the present arrangement of our planet, silently destroying by 
chemical and mechanical action even the hardest rocks, and 
transporting the detritus to distant points, and even to the 
sea, to be consolidated into future strata, in the slow but 
unremitting mutations to which all matter is exposed. 
The aqueous agents by the aeration of rocks and mechani- 
cal formation, are incessantly at work, reducing the inequali- 
ties of the surface to a level, and in circulating water from 
sea to land and land to sea. The atmosphere is the great 
communicatinor ao^ent between the surface of the ocean and 
that of the land, and by means of evaporation we are supplied 
with water. The aqueous vapor in the air of this part of the 
country, though variable with the season of the year and 
other circumstances, will average at least 1 per cent, or 3| 
grains in each cubic foot. Highly elevated lands, which 
attract in proportion to their volume and density a larger 
quantity of this vapor than do the lower hollows and plains? 
are much exposed to atmospheric vicissitudes, — to frost, alter- 
nations of moisture and desiccation, to cold and heat. The 
ordinary action of beating rain is great, but water possesses 
the mechanical property of expansion during congelation, 
which, when it occurs in fissures and crevices of the rocks, 
exerts a power capable of rending asunder the heaviest and 
most compact strata, and thus exposes larger surfaces to be 
operated upon, so as to facilitate their ultimate reduction. 
But besides, water is the great, the universal solvent, and 
aided by the carbonic acid of springs, and that contained in air, 
which though variable, on an average amounts to about 1 part 
in 2000, it acts particularly on the earthy elements of stone. 
From these assaults the surfaces of all matter have no escape. 
The Millstone Grit is durable, as the abbey walls of Kirk- 
