John Le Conte—Sound-Shadows in Water. rif 
The point of especial interest is that these joints have been 
developed in post-Glacial time within a series of strata not per- 
ceptibly indurated, and which repose undisturbed: in the place 
where they were deposited. The strata are nearly horizontal 
and their inclination of less than one-half a degree, northward, 
is presumably the original slope of the bottom upon which they 
were thrown down. Within the basin of the ancient lake there 
have been small orographic movements since the Glacial Epoch 
and a few faults are known to have taken place but no evi- 
dence has been found of such disturbances in the vicinity of 
this locality. The lake-basin has been the scene likewise of 
post-Glacial voleanic eruption but the nearest locality is eighty 
miles distant from the Old River Bed. 
the subject in the establishment of the true theory of the origin 
of the structure. 
Art. IIL—On Sound-Shadows in Water ; by JoHN LeContes. 
cles placed in the route of the sound-waves, being elastic, pro- 
pagate, more or less perfectly, the sonorous vibrations of the air 
through their thickness; so that, under these conditions, it is 
similar to producing a light-shadow by means of a transparent 
or translucent body. 
2. even in cases in which the sound-vibrations in air are 
hot sensibly transinitted through the intervening obstacle, the 
oundaries of the sound-shadows are necessarily very imper- 
tensity of sound behind it, although quite perceptible, is by no 
8 SO Conspicuous as might be expecte 
