1882 .] 
371 
[Davis. 
St. Laurent, and the farmers of the valley became fishermen ; it 
existed about forty years, when the barrier gave way and the 
lake was drained. 1 
The southern slope of the Himalaya contains a number of 
small, picturesque lakes, such as Naini Tal, a favorite summer 
sanitarium, probably held by land-slip dams ; the cause of the 
slip is the excessive rainfall which softens the rock and soil on the 
steep hill-sides. 2 
C. 10. Beaver Dams . The beavers have contributed many 
small ponds, now mostly transformed to marshes or meadows, to 
the northern part of our country. 3 These, with the two styles or 
lagoons formed by coral reefs are the only species we have to 
note as the result of animal, organic forces; they differ very 
decidedly however, as the coral lagoons are entirely unin- 
tentional on the part of the reef builders, while the beaver-dams 
are the result of well-directed efforts to this end. 
All the Obstruction Basins thus far mentioned have been of the 
Barrier family : the following are Enclosure Basins or hollows left 
from an insufficiency or irregularity of deposit. 
C. 11. Delta Basins. Pontchartrain and the other lakes of 
smaller size near the mouth of the Mississippi stand in hollows 
left by the failure of the river to fill its delta region up to a uni- 
form level. The river is inconstant in its course here as well as 
farther north, but from a different cause ; here the continual accu- 
mulation of fine silt raises the bed and banks of the stream until 
it flows in a channel a little above the adjoining country ; then a 
breach made during a flood-overflow may divert it to one side or 
the other, and in the new course so given, the raising process and 
the breaking away will be repeated: the various lines of flow 
will then be marked by rather higher deposits than the interven- 
ing spaces, as the present form of the delta where it advances 
into the Gulf shows very well, and finally the interlacing of old 
1 Reclus, La Terre, I, 522. * 
2 Ball, On the Origin of the Kumaon Lakes, India Geol. Surv. Records, xi, 1878, 
174-182. Theobald, The Kumaun Lakes, id. xm, 1880, 161-175, considers them of 
glacial origin. See also R. D. Oldham, On the Naini Tal Landslip, 1880, id. xiii, 
1880, 277. 
8 L. H. Morgan, The North American Beaver, Philadelphia, 1868. H. Credner, Peterm. 
Geogr. Mittheil., xv, 1869. 139. 
