34 On the Delta and Alluvial Deposits of the Mississippi. 
and that of Pyrochlore in the vein in granite at Chesterfield, 
that a comparison of the two seems worthy of notice ; premising, 
however, that I have examined too small a quanti ty 0 
the St. 
. Michael’s rock, to arrive at any conclusions of consequence. 
Chesterfield, Massachusetts 
vein of granite in mica slate, 
lying wack in the direction of the slate, 
obliqu alk ae vein of the fi 
lowing c 
Albite--chiet ingredient; the fei eg ap- 
pearing mostly a n the cen- 
_tral part of the vein. 
Sa dasha ae ed, green, blue and black, 
es aiscys en in small 
ee Scent y very moder. 
Pyroc ot (eolumbate off ana igisinn 
ichael's, Azores. 
Contents of voteent ic boulders. 
Albite—chief co ituent. 
Tourmaline : Le soneecble 
quantity. Crystals sma 
Mica—a trace 
Columbate of lime, the anne described 
in this paper, gan, very small, but 
containing a aan ranium 
Titanium—Pyrrh sina very small. 
There is anothe ic rock frow 
this locality, in which 
Perch by Ryakolite, and the octa unt 
ti “ee — rrhite by a titanium mineral of a 
Damo ferent orm 
Columbite ‘extremely rare. 
Uraniom 
‘jtanium—a trace sometimes in the Py- 
rochlore. 
Professor precy tle does ty ba the 
bearings of th leas and I 
have not eather the local 
Art. IV.—On the Delta and Alluvial Deposits of the Missis- 
sippt, and other points in the Geology of arent America, ob- 
served in the years 1845, 1846; by C. Lyrun 
Tue delta of the Mississippi may be defined as athe part of 
the great alluvial plain which lies below, or to the south of the 
branching off of the highest arm of the river, called. the Atcha- 
falaya. "This delta is about 13,600 square mi miles in area, and ele- 
vated from a few inches to ten feet above the level of the sea. 
The greater part of it protrudes at the Gulf of Mexico beyond 
the general coast line. The level plain to the north, as far as 
Cape Girardeau in Missouri nbow the junction of the Ohio, is 
of the same character, including, according to Mr. Forshey, an 
area of about 16,000 square miles, and. is, sotenpcln gee larger than 
the delta. It is very variable in width i 
miles wide ; at Memphis 30; at the mouth of the White River 
80, and contracting again farther south, at Grand Gulf, to 33 
miles. ‘The delta and alluvial plain rise by so gradual a slope 
from the sea as to attain, at the junction of the Ohio, (a distance 
of 800 miles by the river,) an elevation of only two hun 
* Abstract of a discourse before the British Association, Sept. 14, 1846, from the 
Athenzum, for September 26. 
