1882 .] 
3T9 
[Davis. 
series, and none of them, so far as I have been able to observe 
or learn, are in true rock basins. They owe their existence to ice- 
occupation during the deposit of the drift that is always found 
plentifully about them; the fact that their longer measures 
generally agree with the trend of glacial striae does not prove 
them to be the result of glacial erosion, but suggests that the val- 
leys in which they lie and whose direction determines their longer 
axis, run with the general slope of the country, and consequently 
with the direction of motion taken by the great glacier. The 
ponds and the striae agree because both are the result of the same 
cause. 
If a review be now made of the Glacial Erosion, Moraine and 
Drift Barrier, and Drift Lakes, it will be seen that much less impor- 
tance is attributed to ice erosion than to its indirect effect in form- 
ing lakes by obstruction and preserving them by occupation : I 
consider the latter causes far more important than the former, 
and regard them as giving rise to much larger and more numer- 
ous basins. 
C. 14. Crater Basins . The volcanic lakes already described, 
(A 6, B 4) probably have certain points in common with this 
species, as it is impossible in most of the examples given to deter- 
mine the ratio between the effects of subsidence and of explosive 
eruption. Basins of volcanic subsidence may include those of 
considerable size, where there is no evidence to show that any 
material has been thrown out by eruption. Pit-crater basins are 
small, abruptly bordered cavities, with evidence usually of some 
eruption, but very possibly aided by subsidence ; the type-exam- 
ples have no distinct rim-wall enclosing them, but are sunk below 
the general surface of the surrounding country. Crater basins 
now to be considered have distinct walls of lava or ashes, sloping 
gently on the outer side and more abrupt within, and although 
even here subsidence of the central parts may have played an 
important part in the production of the basin, the more evident 
cause of the lakes they contain is the existence of the oval or cir- 
cular barrier wall, and therefore they are placed under our third 
class of Obstruction Basins. 
Excellent examples of these occur in the volcanic district about 
Rome and Naples: near the former are Albano and Nemi; near 
