E. W. ifilgard— Later Tertiary of the Gulf of Mexico. 68 
About that time, as E. A. Smith’s late observations show, 
the Peninsula of Florida emerged from the water, apparently 
in the prolongation of the upheaval which traverses the State 
of Georgia from Atlanta to its southeast corner, forming the 
great “divide” between the rivers flowing directly to the 
Atlantic, and those tributary to the Gulf This axis of up- 
heaval m informed b r. Loughridge, is marked by 
numerous and very long trap dykes, running parallel to it in 
‘ the metamorphic region of the State. As Dr. Smith has ob- 
served, there is a distinct ridge or ‘“‘back-bone” of Florida, 
formed of the Orbitoides limestone, that does not lose itself 
entirely until the Everglades are reached. On the Florida shore, 
the Vicksburg rock is mostly covered to a greater or less depth 
by the Quaternary coralline rock, though outcropping at 
Tampa and a few other points. 
Subsequent to this upheaval, the Miocene and Pliocene beds 
were deposited on the Atlantic side of the peninsula, as they 
were on the rest of the Atlantic coast. Meanwhile, what 
happened on the Gulf side? 
As we have seen, the Grand Gulf beds were being deposited 
during that time, or a part thereof, in the axis of the Mississippi 
trough, and all around the Texas shore to the Rio Grande, and 
doubtless beyond. Toward the east, these beds “run out” on 
or about the Perdido River, on the line between Alabama and 
Florida. 
A glance at the map of the Gulf soundings will show that 
this places the western line of the outcrop of the Vicksburg 
rocks exactly in the prolongation of the edge of the great 
submarine border plateau outlined by the ‘“ 100-fathom line,” 
from which there is such a sudden descent, all around the Gulf, 
Into deep water. 
It may be premature to infer from this coincidence, that if 
great shelf furnishes, as it seems to me, an explanation of the 
“Grand Gulf” rocks on the mainlan 
Mississippi Valley are proven to have been greater than on 
either side of the same; in other words, that it is, and has 
en, an axis of weakness and disturbance. As to the extent 
of its vertical movements in later Tertiary and Quaternary 
times, I have elsewhere shown that it cannot have been less 
than 900 feet between the time at which the great drift floods 
carried the northern pebbles to the Gulf shore, and that at 
which the loess of the Mississippi Valley was deposited. For 
we find the drift pebbles at a depth of 450 feet below the 
