638 AMERICAN ASSOCIATION ADVANCEMENT OF SCIENCE. 
Webster condenser, but of 110°, perfectly achromatic and adjustable for 
thickness of the glass slide, and mounted over a graduating diaphragm, 
a pair of shutters for an adjustable horizontal slit (one of them, or an- 
other single shutter, capable of moving independently for oblique illum- 
the ordinary investigation in natural history would consider the combi- 
nation rather as a ET part of their stand than as an accessory to 
be sometimes used with it 
Dr. WALKER of ^ Orleans, La., read a paper prepared by Professor 
EuGENE W. HILGARD, State Geologist of Louisiana, on ** The Upper Delta 
Plain of the Mississippi." The paper is one of a series by the same au- 
thor, the preceding ones having treated of the older formations which 
characterize the geology of the Great Enlargement of the "pss Gulf 
Basin, of which the Mississippi River, below Cairo, forms the 
delta or alluvial deposits proper, cover the older formations 
paratively slight depth only, the river running on paludal pdt w 
then on an ancient sea bottom, of corresponding (late quaternary) age, 
from above New Orleans to near its mouth, It thus appears that Artesian 
bores in the vicinity of New Orleans, tubed through the (chiefly marine) 
seen to have swept over the southern coast with sufficient force to trans- 
port pebbles of five to six ounces weight from far distant regions, the 
nearest being Tennessee and Arkansas. This great eroding agent seems 
also to have so cut and worn the older formations into ridges and chan- 
nels, that the overlying ones vary greatly in thickness, while level at the 
surface. The singular phenomena known as the New Orleans Gas Wells, 
are also mentioned. When bores were sunk for water, the gas rushed up 
with such force as to carry up several cart loads of sand in a single night, 
and when the gas became ignited, it was extinguished with great dif- 
culty. 
An abstract of a second paper by the same author, “On the Mudlumps 
of the Passes of the Mississippi,” was given by Prof. J.E.Hmaarp. The 
Mudlumps are islands formed by upheavals of the bottom, off the mouths 
of the Passes, inside the bar. They often rise in mid-channel, obstruct- 
ing navigation and diverting the current, and at times bringing up ob- 
jects long ago lost from vessels. They form a number of pretty large 
islands, especially near the mouth of the South-west Pass. On them we 
frequently find springs of liquid mud, accompanied by bubbles of com- 
bustible gas; these springs often exhibit all the phenomena of mud vol- 
canoes — extensive cones of mud, with an active crater in the middle. 
Most of the material of the Mudlumps seen above water, bears evidence 
of having once belonged to active cones, now extinct. 
MEE USt eem EE TN S 
cue cani e A EMI e 
