Ie) 
Ful 1886] 
NATURE 
269 
should do away with the acrid yellow character of our 
fogs. But the mists due to the position of London on the 
estuary of a large river, would remain to the same extent 
as now, and there would still be the same amount of 
sulphurous acid given off into the air to be precipitated with 
the rain as sulphuric acid, and carry on its work of destruc- 
tion on building stones and mortar. One cubic foot of coal- 
gas produces on combustion 0°2 to 0’5 grains of sulphurous 
acid, so that the amount evolved would continue to be, as 
now, enormous. Still the air would be deprived of its 
sooty particles to a great extent, and the old familiar 
features, characteristic of grimy London, might in time 
disappear. The carbonic acid which is the chief product 
in the combustion of coal-gas, is diffused at once into the 
general body of the atmosphere, and the marvellous 
rapidity with which this is effected is revealed to us 
when we know that the air of our open streets and parks 
differs only by the most minute quantities—if at all— 
in its contained carbonic acid, from the air of the 
mountains or the sea. 
THE TOPOGRAPHIC FEATURES OF LAKE 
SHORE St 
Introduction 
“THE play of meteoric agents on the surface of the land 
is universal, and there is a constant tendency to the 
production of the forms characteristic of their action. All 
other forms are of the nature of exceptions, and attract 
the attention of the observer as requiring explanation. 
The shapes wrought by atmospheric erosion are simple 
and symmetric, and need but to be enumerated to be 
recognised as the normal elements of the sculpture of the 
land. Along each drainage line there is a gradual and 
gradually increasing ascent from mouth to source, and 
this law of increasing acclivity applies to all branches as 
well as to the main stem. Between each pair of adjacent 
drainage lines is a ridge or hill standing about midway 
and rounded at the top. Wherever two ridges join there 
isa summit higher than the adjacent portion of either 
ridge ; and the highest summits of all are those which, 
measuring along lines of drainage, are most remote from 
the ocean. The crests of the ridges are not horizontal, 
but undulate from summit to summit. There are no sharp 
contrasts of slope; the concave profiles of the drainage 
lines change their inclination little by little, and they merge 
by a gradual transition in the convex profiles of the crests 
and summits. The system of slopes thus succinctly in- 
dicated is established by atmospheric erosion under the 
general law of the interdependence of parts. It is the 
system which opposes the maximum resistance to the 
erosive agents. 
The factor which most frequently, and in fact almost 
universally, interrupts these simple curves is heterogeneity 
of terrane or diversity of rock texture. Different rocks 
have different powers of resistance to erosion, and the 
system of declivities which, under the law of interde- 
pendence, adjusts itself to diversity of rock texture, is one 
involving diversity of form, Hard rocks survive, while 
the soft are eaten away. Peaks and cliffs are produced. 
Apices are often angular instead of rounded. Profiles 
exhibit abrupt changes of slope. Flat-topped ridges 
appear, and the distribution of maximum summits becomes 
in a measure independent of the length of drainage lines. 
A second factor interrupting the continuity of erosion 
profiles is upheaval, and this produces its effect in two 
distinct ways. First, the general uprising of a broad tract 
of land affects the relation of the drainage to its point of 
discharge or to its base level, causing corrosion by streams 
to be more rapid than the general waste of the surface, and 
producing canons and terraces. Second, a local uprising 
* From a paper by Mr. G. K. Gilbert in the “Fifth Annual Report of the 
Geological Survey of the United States for 1883-84.” (Washington, 1885.) 
by means of a fault produces a cliff at the margin of the 
uplifted tract, and above this cliff there is sometimes a 
terrace. 
A third disturbing factor is glaciation, the cz7gues and 
moraines of which are distinct from anything wrought by 
pluvial erosion ; and a fourth is found in eruption. 
The products of all these agencies except the last have 
been occasionally confused with the phenomena of shores. 
The beach-lines of Glen Roy have been called river 
terraces. The cliffs of the Downs of England have been 
ascribed to shore waves. Glacial moraines in New 
Zealand have been interpreted as shore terraces. Beach 
ridges in our own country have been described as glacial 
moraines, and fault terraces as well as river terraces have 
been mistaken for shore marks. Nevertheless, the topo- 
graphic features associated with shores are essentially 
distinct from all others ; and when their peculiar characters 
are understood there is little occasion for confusion. It 
is only where the shore record is faintly drawn that 
any difficulty need arise in its interpretation. In 
investigating the history of Lake Bonneville and other 
Quaternary water bodies of the Great Basin, the writer 
and his assistants have had constant occasion to distin- 
guish from all others the elements of topography having 
a littoral origin and have become familiar with the criteria 
of discrimination. Their endeavour to derive from the 
peculiarities of the old shore lines the elements of a 
chronology of the lake which wrought them, has led them 
to study also the genesis of each special feature.! 
In the discussion of shore phenomena there is little 
room for originality. Not only has each of the elements 
which go to make up the topography of a shore been 
recognised as such, but its mode of origin has been ascer- 
tained. There appears, however, to be room for a 
systematic treatment of the subject in English, for it is 
only in continental Europe that its general discussion has 
been undertaken. The writings of Elie de Beaumont 
include a valuable contribution,’ and Alessandro Cialdi 
has devoted a volume to the motion of waves and their 
action on coasts.® These cover a large portion of the 
ground of the present essay, but treat the subject from 
points of view so diverse that the essay would be only 
partially superseded by their translation. The title of a 
work by H. Keller (“Studien itiber die Gestaltung der 
Sandkiisten”) indicates another discussion of a general 
nature, but this I have not seen. American and British 
contributions are contained chiefly in the reports of en- 
gineers on works for the improvement of harbours and 
the defence of coasts. The most comprehensive which 
has fallen under my eye, and one, at the same time, of the 
highest scientific character, is contained in the annual 
report of the United States Coast Survey for 1869, where 
Prof. Henry Mitchell, in treating of the reclamation of 
tide lands, describes the formation of the barriers of sand 
and shingle by which these are separated from the ocean. 
It is proper to add that the writer became acquainted 
with these works only after the body of this essay was 
prepared. The objective studies on which his conclusions 
are based had been completed, and the discussion had 
acquired nearly its present shape before he became aware 
of the extent of the affiliated literature. His conclusions 
have, therefore, the quality of independence, and, so far 
as they coincide with those of earlier writers, have a 
corroborative value. 
The engineering works whose construction has led to 
local investigations of shores are chiefly upon maritime 
coasts, where tides exert an important influence, and the 
literature of lake shores is comparatively meagre. It is 
Tt Partial outlines of the subject have been presented by the writer in 
connection with various accounts of Lake Bonneville, and a fuller outline 
was published by Mr. I. C. Russell in a paper on Lake Lahontan in the 
“Third Annual Report of the Geological Survey.” 
2 ‘‘ Lecons de géologie pratique ;’” tome premier ; septi¢me lecon, ‘* Levées 
de sable et de galet,”’ pp. 221-52. 
3 “Sul moto ondoso del mare e su le correnti di esso specialmente su 
quelle littorali pel comm ’’ Alessandro Cialdi. Roma, 1866, 
