INTERPRETATION OF FOSSIL PLANTS 
15 
of Nyssa common in the Eocene deposits of Texas, which, in 
nearby localities, all lie flat in water-laid mud (clay lenses), or 
are jumbled on end and at every other angle in wind-blown sand.® 
A second illustration may be taken from the Late Cretacic 
Cheyenne standstone of southern Kansas.’’’ Here in a limited area 
the sands contain curled-up, coriaceous leaves and fully dried 
Sequoia cones with distended scales; whereas in small clay lenses 
the leaves are flat and the remains of more delicate plants, such as 
ferns, are present in a recognizable state. 
Aside from their botanical facies, which is obvious, wet, low¬ 
land tropical climates will be characterized by small, entire, cor¬ 
iaceous leaves, or by compound leaves, with small, entire, coria¬ 
ceous leaflets. A combination of humidity and shade in general 
results in large and membraneous leaves. Insolation, either of 
strand, dunes or uplands, tends to develop the usual evaporation 
limiting devices for water conservation; and undue amounts of 
salts in the substratum, even in abundant water, results in phy¬ 
siological dryness, difficult to distinguish from true xerophytism. 
Seasonal changes, either of temperature or moisture, wdll be re¬ 
flected not only by the so-called rings of exogenous woods, but 
also be inferred by the vertical distribution of leaves where sedi¬ 
mentation is continuous, truly deciduous forms being confined to 
the temperate zones. Referring to the processes of fossilization, 
woods are commonly lignified (carbonized) in clays or swamp 
deposits. Silicifi'cation is usually a concomitant of sandy deposits, 
and calcification takes place in the presence of appropriate salts 
of lime in solution. Both silicification and calcification are fre¬ 
quently secondary, how'ever, and due to the circulation of appro¬ 
priately charged groundwaters. It is not uncommon to find pros¬ 
trate logs with one end silicified in sand and the other end lignified 
in clay, or to find lignified logs with the heart-wood silicified and 
the sap-wood lignified. Ferruginization of wood may be accom¬ 
plished by changes to limonite, or other iron salts, but is usually 
produced by the formation of the carbonate under swamp condi¬ 
tions. Replacement by the salts of the other metals are rare and 
take place only under special conditions. 
Plant material showing structural details, aside from petrified 
woods, are found at relatively few geological horizons. When 
6 U. S. Geol. Surv., Prof. Paper 92 (in press). 
7 Idem., Prof. Paper 129, pp. 199-225, 1922. 
