390 Harper: FACTORS INFLUENCING DISTRIBUTION 
gence the larger rivers of the coastal plain cut their channels 
entirely through the Lafayette deposits into the older strata below, 
and in most of the resulting gorges the Columbia sands were sub- 
sequently deposited. 
Like the Lafayette, the Columbia covers a large part of the 
coastal plain, extending from Long Island to Mexico, and up the 
Mississippi and Ohio rivers to Indiana. In the Middle and South 
Atlantic States it covers the whole country near the coast, but usu- 
ally does not extend up to the inland edge of the coastal plain like 
the Lafayette. In southwestern Georgia and Alabama, however, 
it is mostly confined to the river valleys, the divides here having 
been above the sea during the Columbia submergence. In Texas, 
while the main body of the Columbia only covers the seaward por- 
tion of the coastal plain, long narrow arms of this formation extend 
several hundred miles up the larger rivers, far beyond the inland 
edge of the costal plain. Similar extensions are found in Arkansas 
and the Indian Territory. On the peninsula of Florida the Colum- 
bia formation has not been traced south of latitude 28°. 
The geographical distribution of our two species of Z¢ wxoaium 
is intimately related to that of the two geological formations just 
mentioned, as will now be shown. 
The range of 7. distichum is given by Professor Sargent™ as 
follows: “From southern Delaware * * * southward near the 
coast to the shores of Mosquito Inlet and Cape Romano, Florida, 
through the coast region of Gulf States to the valley of the Devil 
River in Texas, and through Louisiana and Arkansas to south- 
eastern Missouri, eastern Mississippi and Tennessee, western and 
northwestern Kentucky, southern Illinois, and Knox county ™ 
southwestern Indiana.” 
The range thus given of course includes that of 7. imbricar™ 
also, but according to my theory 7. distichum would be confined 
to the comparatively small but widely distributed area from which 
the Lafayette formation is absent, such as near the coast, along the 
larger rivers, and on the peninsula of Florida. 
Among the extreme points of the range of 7. distichum may - 
mentioned the following : 
 -* Sylva N. A. to: 153. ‘1806. 
