oF TAXODIUM DISTICHUM AND RELATED SPECIES 389 
noted will probably disappear on closer investigation. It may be 
_ briefly stated as follows: 
Laxodium imbricarium always grows over the Lafayette forma- 
tion; Taxodium distichum never. 
_ Furthermore, 7. zméricarium seems to occur only on Lafayette 
which is overlaid by a thin deposit of the Columbia formation, and 
I. distichum is often, if not usually, found on the Columbia also, 
but where this formation is absent it may grow directly on the 
older strata. 
The Lafayette and Columbia formations have been thoroughly 
discussed by W J McGee,* but a brief outline of them here may be 
of interest to those who have not time to read the one hundred 
and seventy-five quarto pages of his monograph. 
The Lafayette, the older of the two, is (at least throughout 
most of the region in which Zaxodium imbricarium occurs) a de- 
posit of sandy clay, reddish or yellowish in color, varying consid- 
erably in thickness, lying unconformably on the mesozoic and 
Cenozoic strata over a vast area of the coastal plain of the eastern 
United States. It is supposed to have been laid down just before 
the Glacial period of the North, during a submergence of the 
coastal plain estimated to have lasted about 60,000 years. 
The Columbia formation, in the region under consideration, 
consists almost entirely of sand. It is supposed to have been 
deposited during a much shorter period of submergence of the 
Coastal plain contemporaneous with or subsequent to the glacial 
period. It is always above the Lafayette where the two come in 
Contact, and is rarely too thick for the roots of trees to penetrate 
through it into whatever strata may be beneath. 
In a general way the terrane of the Lafayette may be said to 
Coincide with the present coastal plain from Maryland to Texas, 
extending up the Mississippi Valley to Illinois, with the following 
€xceptions, It is not known to cover any portion of Florida ex- 
cept the extreme north, the peninsula having perhaps been too far 
off shore during the Lafayette submergence to receive any of these 
Sediments. Neither is this formation known to extend to the pres- 
€nt shore line at any point. 
During a period of elevation succeeding the Lafayette submer- 
*Ann. Rep. U. S. Geol. Surv. 12!: 347-521. pl. 32-41. f. 28-72. 1891. 
