604 GEOLOGY. 



is chiefly taking place in high latitudes, though it is also taking place 

 in favored situations in low latitudes. These situations, however, 

 are rather rare and local, while peat formation in high latitudes covers 

 great areas. A high temperature, while it promotes a luxuriant growth 

 of vegetation, also promotes, in an even higher degree, decomposition, 

 under usual conditions. 



The characteristic foliage of a warm, moist atmosphere takes the 

 form of broad leaves with a thin epidermis and with many and large 

 breathing pores, affording abundant facilities for the transpiration 

 of moisture. The dominant plants of the Coal flora, notably the char- 

 acteristic lepidodendrons and sigillarias, have narrow leaves with the 

 breathing pores confined to deep furrows on the under side, and even 

 then, in some cases, protected by hairs, devices that are common when 

 the plant must guard itself against too rapid expiration of moisture, 

 devices technically known as xerophytic. These have been care- 

 fully noted in the preceding descriptions, because of their possible 

 significance. It was also observed that most of the leaves of the forest- 

 trees were protected from too great action of the sunlight, by palisade 

 cells on the upper sides of the leaves. It was noted further that 

 most of the trees were clothed by unusually thick corky bark, as 

 though protection from external conditions was needed. It has been 

 suggested, indeed, that this bark was succulent, rather than corky, but 

 it is the part best preserved, and, according to Dawson, often makes .up 

 a large part of the coal, because the wood and the foliage more largely 

 perished, facts hardly consistent with a succulent nature. The remark- 

 able preservation of the leaf scars is interpreted, on the one side, to 

 mean an extremely rapid growth and quick burial, and on the other 

 side, to mean a texture very resistant to decomposition, and an atmos- 

 phere unfavorable to decomposition, by reason of dryness or coolness. 

 Taken as a whole, the thickness of the bark, its superior resistance 

 to decomposition, the form of the leaves, presenting small surfaces 

 rather than expansive ones, with their strong sclerenchymatous tis- 

 sues, their palisades, and their restricted and protected stomata, give 

 a pronounced xerophytic aspect to the overgrowth made up of lepi- 

 dodendrons, sigillarias, calamites, and cordaites. It is not equally so 

 with the undergrowth, the ferns, and the sphenophylls, but this should 

 not be expected of shaded plants, unless the aridity was extreme. 



The force of the inference from the xerophytic aspect of the over- 



