CLIMATIC CONDITIONS . 473 



which today favor arboreal or herbaceous forms held true for those of the 

 sub-Carboniferous^ yet in a general way this must be true, since arboreal 

 forms are developed by competition for sunlight among those plants whose 

 roots are able to penetrate deeply for groundwater and whose superstruc- 

 ture does not die each year. The present grasses, on the contrary, and by 

 analogy the grass-like forms of earlier floras, are favored by light and 

 fairly frequent rains during a growing season, which may be brief and 

 after which the exposed portion of the plant dies. The presence of 

 deeper-seated stores of groundwater is of no moment to the prairie types 

 of vegetation. It is these relations of rainfall and groundwater which, as 

 Schimper points out, determine the distribution of woodland and grass- 

 land. Over many regions the competition between the two types of vegeta- 

 tion is, however, very close, resulting in a mixed or park-like formation. 

 In Paleozoic times it may very possibly have been true that specialized 

 drought-resisting arboreal forms had not yet come into existence, and 

 the climatic relationships of herbaceous and arboreal societies may have 

 been then more sharply drawn than in modern vegetation. At least, 

 evolutionary progress would be in the direction of an obscuring of such 

 relationships. 



Turning to the vegetal evidences found in the Mauch Chunk shales 

 and which have been described previously, it was noted that, although 

 the impressions of an herbaceous vegetation were not uncommon, no 

 impressions of logs, such as are so common in the overlying Pottsville, 

 have ever been observed. The various markings, furthermore, show small 

 forms, such as could well be produced in a single growing season. The 

 downward striking roots point to a poverty in superficial moisture, and 

 the absence of an arboreal vegetation suggests a deficiency during the 

 growing season of more deep-seated waters. Even in arid climates, if 

 the streams flow during the whole year or the groundwater is within the 

 reach of the roots of trees, the streams are fringed with shrubs and trees. 



The evidences of the vegetal impressions point, therefore, toward a 

 climate such that in the lower portions of the river courses the streams 

 flowed only during a part of the year. During another part of the year 

 the surface dried up and the groundwater sank to depths where the roots 

 of the early trees could not penetrate. These evidences therefore point 

 to an arid or semiarid climate. 



The amphibian footprints might be thought to weigh against this 

 conclusion, but the force of such a criticism may be weakened by a con- 

 sideration of the following possibilities : It is to be noted that the present 

 dipnoi, ceratodus and protopterus, have only escaped extinction in regions 

 with a marked dry season, the ceratodus living in two Australian rivers 

 which are in winter reduced to pools, the protopterus living in certain 



XLI — Bull. Geol. Soc. Am.j Vol. 18, 1906 



