4. Some Points in Geology. 
Salter discerns traces of these old rocks in Wales. In fact, all the 
metamorphic rocks are being referred to either one or other of the 
great rock-systems or groups of sand, mud, and limestone that have © 
succeeded one another with the changing outlines of land and sea; 
thus the Cretaceous limestones, clays, and sands (our Chalk, Gault; 
and Greensand), unaltered in England, are changed to marble, 
slate, and crystalline schist in Greece and elsewhere; and in Cali- 
fornia, as Mr. Whitney tells us, these and other Secondary strata are 
metamorphosed into the gold-bearing mountain-rocks. There is no 
evidence now of an ‘azoic’ period, nor is there a separate under- 
lying ‘metamorphic system,’ such as old school-books teach; and 
thus one hindrance in threading the maze of mountain-structure is 
removed. 
Aswe have now more correct views than heretofore as to the 
internal structure of mountains and continents, so we may regard 
as approximately true our notions of the methods by which the tops 
and flanks of ridges have been chiselled out of the upraised strata, 
hard and soft, crystalline and earthy, bent, folded, and overturned, 
as the case may be; and by which the slopes and plains were laid 
out, and channelled with converging drainage-lines, that begin at 
the crest of the neighbouring mountains and reach to the sea, with 
snow-fields, glaciers, torrents, streams, tarns, lakes, rivers, and 
deltas characterizing this and that portion of their course. But in 
both cases we want very much information as to details; as to 
the different stages of the various operations, and as to the parti- 
cular share taken by the several agents in the work. Of late the 
action of Glaciers has been a favourite study, and the actual 
work they now perform has been taken as a measure for the enor- 
mous effects of their transporting and grinding power when frost, 
snow, and ice reigned supreme in the northern hemisphere and on 
all high ranges. The frost breaks the surface of the rock, the 
glaciers carry the fragments along gorges and valleys, grinding 
those channels wider and deeper; and, removing the débris as far as 
they reach, drop it with icebergs into the sea, or, in milder climates, 
leave some of it as moraines, and give up a portion to the under- 
working river that runs from its foot. The glacier, like water, has 
followed a drainage-line ;—did this channel first begin in the wear 
and tear of waves on an uprising shoal, or is the furrowed ridge 
itself a sharp-edged remnant of a great plain, worn smooth by waves, 
and then left to be eaten into by air and water, chemically and 
mechanically, until by little and little, by the wedging and grinding 
of ice, by storm and torrent, nearly all has been removed? In both 
cases (and they would each depend, in all their modifications, on 
length of time and rate of uprise, whilst the land was subjected to 
wave-action ), it is possible that the drainage-lines would have their 
directions given them by slight inequalities of the surface, and dif- 
ferences in the texture of the rock ; but it is argued that not only must 
the many cracks and faults traversing the strata (more than ever 
will be shown on any map), and the many lines of weakness along 
the edges of tilted and folded beds, have given primary direction to 
