192 GEOLOGICAL SURVEY OF NEW JERSEY. 



which prevails over most of the tension zone, and its abrupt 

 limitation where this ceases. Querciis Phellos and Q. nigra are, 

 without doubt, limited in their distribution by the same cause, 

 and we are justified in assuming that if the soil conditions, or, 

 what is the equivalent, the geological formation in which they 

 grow, had a further northward extension, the species mentioned 

 would be found upon it.* 



When the chemical characteristics of the soil of each zone are 

 studied, it becomes easy to understand why the vegetation in 

 each is so different. In the deciduous zone the rocks are par- 

 tially weathered and disintegrated for a considerable depth, and 

 this disintegration is unceasingly going on, constantly adding 

 new material to the soil from the variety of the mineral con- 

 stituents in the rocks. New soil is constantly being made and 

 the old soil being renovated, so that plant food is in process of 

 manufacture all the time. The mechanical character is also 

 such that it is capable of retaining moisture for a considerable 

 period, which is a valuable factor in periods of drought. 



On the other hand, in the coniferous zone the rock has long 

 been almost completely disintegrated, and as it is practically 

 composed of but one constituent, quartz, which is of little or no 

 value for plant food, any further disintegration is incapable of 

 yielding any other element, and but little is added to the soil 

 which could serve to support vegetation. Further than this, in 

 many places no rock disintegration is going on, but, on the con- 

 trary, rock is in actual process of formation. Sandstones and 

 conglomerates are being formed by cementation with limonite, 

 and where this occurs a hard layer results which limits the 

 downward growth of roots, while if the conditions are such that 

 an open, porous sandy soil prevails, it becomes impossible for 

 water to be retained in it, and an arid sand barren is the 

 result. 



One thing, however, as previously hinted, should be borne in 

 mind in regard to many typical or characteristic species, which 

 is that they do not always exist in a region by reason of the 

 environment being the most favorable for them, but because the 



* In this connection the following reference may be found of interest : 



i. "On the Existence of a Peculiar Flora on the Kittatinny Mountains of Northern New Jersey.'' 

 N. L. Britton. Bull. Torrey Bot. Club, Vol. XI (1884), p. 126. 



2 " Plant Distribution as a Factor in the Interpretation of Geologic Phenomena, &c." Arthur Hoi- 

 lick. Trans. N. Y. Acad Sci. Vol. XII (1893), p. 189. 



