JULY 28, 1899. ] 
time makes a sheltered harbor. As the 
sandspit continues to grow, its point event- 
ually joins the mainland, and the harbor is 
converted into a closed lagoon. This com- 
plete cycle of changes has taken place at 
Greytown during the last century and a 
half, as shown by the early maps of that 
portion of the coast. The cycle has also 
been repeated at the same point several 
times previous to the one of which there is 
documentary evidence, giving rise to the 
several distinct lagoons which occur inland 
from the one last formed. 
The surface of the San Juan delta-plain 
is diversified by occasional hills which were 
at one time islands fringing the coast, and 
also by numerous lakes and lagoons due to 
the uneven distribution of the sediment. 
At its inner margin it abuts against the 
foothills or merges with the broad flood- 
plain of the river. 
The San Juan leaves the lake practically 
clear, and most of the sediment which it 
delivers at its mouth is received from two 
large southern tributaries, the San Carlos 
and the Sarapiqui. These have their source 
upon the slopes of the Costa Rican volea- 
noes, and bring down vast quantities of 
black voleanie sand. Below the mouth of 
the San Carlos the trunk stream carries 
more and coarser sediment than any of the 
smaller tributaries. It has, therefore, built 
up its flood-plain more rapidly than the 
tributaries, and the latter are dammed, 
forming extensive lagoons in the side val- 
leys. 
The recent volcanic activity in this re- 
gion has given rise to two series of vents, 
having a very striking linear arrangement. 
The southern series extends diagonally 
across the isthmus, in northern Costa Rica, 
terminating near the Pacific in the extinct 
voleano Orosi. The materials extruded 
from these vents have built up the massive 
mountain range which forms the southern 
border of the Nicaraguan depression. The 
SCIENCE. 101 
second series of vents extends northwest- 
ward from Madera, on an island in Lake 
Nicaragua, to Coseguina, on the Gulf of Fon- 
seca. Between Madera and Orosi, the prox- 
imate ends of the two lines, is a gap of 
about 30 miles. The northern vents were 
at first submarine, extending ina line nearly 
parallel with the former coast. They have 
built up a broad, gently-sloping plateau, 
from which rise, singly and in groups, many 
symmetrical voleanic cones. Most of these 
vents are extinct, while a few have been in 
eruption since the Spanish conquest, but are 
now quiescent. The older cones have suf- 
fered considerable modification by erosion, 
while the newer ones, and also the pla- 
teaus, retain, in a large measure, their orig- 
inal constructional forms. 
The rocks of the Nicaraguan depression 
are, so far as known, Tertiary and later. 
They include both sedimentary and igneous 
formations, though the latter greatly pre- 
dominate. The strip of land between Lake 
Nicaragua and the Pacific, southward from 
a point opposite the Island Zapetero, is 
composed chiefly of sandstone and shales, 
with some beds of limestone, which Dr. 
Dall pronounces to be of Tertiary (Oligo- 
cene) age. The sandstones contain a large 
proportion of volcanic matter and might 
almost be classed as andesitic tuffs. An- 
other area of similar rocks crosses the San 
Juan Valley between Castillo and the Boca 
San Carlos, and may originally have 
been continuous with the area west of the 
lake. With the exception of this small area 
of sandstone, the entire San Juan Valley is 
composed of igneous rocks, including lavas, 
tuffs, breccias and conglomerates. These are 
all, so far as known, of Tertiary age. The 
lavas are chiefly basalts, andesites and da- 
cite. The recent volcanic rocks are chiefly 
andesites, with a few lava flows of basalt. 
The climatic conditions prevailing in this 
region have a very direct connection with 
its physiography and form one of the most 
