REPORT ON FORESTS. 191 



'formations and certain facts of plant distribution, and from 

 whatever point of view we may regard the matter it finally 

 resolves itself into an examination of the soil conditions which 

 are directly dependent upon the structure and composition of 

 the geologic formations from which they were derived. Soil 

 influence is indicated in all the before-mentioned facts of plant- 

 distribution within the state as one of the most potent factors, 

 and in some instances as practically the only one to be taken 

 into consideration. Here it seems pertinent to remark, lest the 

 fact should not be thought of, that the mere name which any 

 geologic formation may bear is of no consequence in this con- 

 nection. The only matter of moment is its lithologic charac- 

 ters, mechanical or chemical, irrespective of age or stratigraphic 

 position. A sandy soil, whether a recent dune deposit or one 

 derived from the disintegration of Palaeozoic sandstone in place, 

 would be of equal importance so far as sand-loving plants are 

 concerned. Similarly a heavy soil, whether of glacial till or 

 Cretaceous marl, might be equally available as a home for 

 species which must love such a mechanical condition for their 

 proper growth, while those which merely require a rocky soil 

 would be indifferent as to whether the rock was Eozoic granite 

 or Mesozoic trap. 



This line of argument, as may be readily seen, infers that the 

 mechanical structure of the soil is of greater importance as a 

 factor in plant distribution than is its chemical composition, and 

 this seems to be the fact. The observations made indicate that 

 the former is the more powerful factor in determining the origi- 

 nal location and distribution of species, while the latter more 

 largely influences their subsequent growth. 



Upon this basis of reasoning we may in part account for the 

 fact that species of the deciduous zone, where the soils are com- 

 paratively rocky or heavy, are able to push their way southward 

 into the clays and marls of the tension zone, and we also note 

 the effect of the chemical composition of the soil on their 

 growth, as indicated by their decrease in size wherever the same 

 species occur, on areas of sand or gravel within the zone. By a 

 similar course of reasoning we may understand the northward 

 extension of species from the coniferous zone, such as Pimis 

 Virginiana, on account of the sandy or gravelly surface-soil 



