98 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
obstructs a valley, and a lake fills the basin behind it. The lake 
outlet is turned across a ridge, where it is actively cutting a gorge by 
which the lake will be soon drained. Inasmuch as one of the lava 
flows runs into a little valley on the coastal plain, it is manifest that 
at least that part of the volcanic accident took place after the plain 
had been elevated and dissected. 
A simple system of describing land forms may be illustrated by 
means of this elementary example. The existing forms of the 
coastal plain may be treated as dependent, first, on the form of the 
plain as it rose from the sea; and, secondly, on such modifications 
as it has suffered since then under the action of weather and water. 
The term initial may be applied to the original form, or to so 
much of it as is still visible on the doabs. The term sequential 
may be applied to the forms that follow from the action of the 
various processes of weathering and washing on the initial form. 
In this case, the smooth doabs still preserve the initial form of the 
plain with insignificant change; the broad valley floors or straths 
of the master streams, and the narrow valleys of the minor streams, 
and the little lateral ravines of the wet weather streams are all 
sequential forms. Ultimately, when the doabs are worn down flat, 
close to sea level, the final form will be reached. Evidently, the 
final form will be attained much sooner on the weak strata of the 
coastal plain, than in the resistant rocks of the oldland mountains. 
The cycle of the latter is much longer than the cycle of the former. 
The surface of the doabs is everywhere so much alike in form 
and quality that villages are located here and there without system. 
Roads run in straight lines for long distances, because they meet no 
obstacles ; but they generally follow ravines in passing from doab 
down to strath. The neighborhood of a master river commonly 
attracts the greatest population of the region. Its strath is gener¬ 
ally fertile, but is exposed to danger from floods. The river comes 
from an extensive system of confluent valleys among the oldland 
hills and mountains, and may lead tribute from mines and quarries, 
forests and upland pastures, to the city near the sea. The city 
serves as the market place for the agricultural products of the plain, 
although these are sometimes not of the best: for where the surface 
is sandy, it may be hardly worth tilling; it may then be left to 
such forest growth as it will bear, or it mav serve for grazing. 
The city is the local center of trade and of commerce from over 
sea, for only through the large river mouth can sea-going vessels 
