Bibliography. 193 



magnesia more than the older ones ; but since all of them contain 

 enough of these and other mineral ingredients for the wants of plants, 

 an excess of some of them cannot sensibly affect vegetation. The 

 mineral composition of this " one soil," by a mean. of numerous anal- 

 yses, the author states to be, in 100 parts, 89.25 of sand, or silicates, 

 and 0.85 salts of lime. 



Thus far we agree essentially with the author. But there are some 

 other facts, which lead us to question whether the statement of this first 

 principle does not require modification. It cannot be doubted, in the first 

 place, that some silicates, (varieties of feldspar, for instance,) are decom- 

 posed, both by atmospheric agencies and by growing plants, with much 

 more facility than others ; and consequently vegetation obtains from some 

 soils the alkali, lime, and silica, which it needs, more easily than from 

 others. In the second place, where an extensive region is underlaid 

 by limestone, a large per cent, of the soil (from 5 to 30 in some parts 

 of Europe) consists of carbonate of lime, and this is usually more easily 

 decomposed than silicates, and will more readily supply carbonic acid. 



The second principle is, that " rocks do not affect the vegetation 

 which covers them.'''' Here again, while we agree with the author that 

 this principle is generally true, far more generally so than is commonly 

 admitted, we would suggest to him whether there are not exceptions to 

 it, too important to be overlooked in stating it. A similar difiiculty we 

 feel in respect to his third principle, that '■'■rocks have not formed the 

 soil which covers them.'''' And we can conveniently, perhaps, state our 

 exceptions to both of them in the same paragraph. 



It is true that what has been called diluvial action, as well as alluvial 

 agency, has mixed up the detritus of different formations, and spread 

 the soil derived from one rock over others. But the great mass of the 

 soil has in very few cases been removed more than a few miles, 

 although single bowlders have been carried hundreds of miles. 

 Hence, when we have gone a few miles from the borders of a forma- 

 tion, the soil is made up almost entirely of its pulverized fragments, 

 removed, it may be a short distance, but still derived from pi'ecisely 

 the same rock as that beneath it. Again, limited formations are 

 sometimes so situated that little if any foreign detritus has been left 

 upon them, as for instance, the trap ranges along Connecticut river. 

 From both these causes it happens, that over a large part of our coun- 

 try, each rock formation of much extent, does present a peculiar soil, 

 which the geologist easily recognizes as derived from the subjacent 

 rock. That above granite and gneiss, for instance, is light colored and 

 sandy ; that from argillaceous and mica slate, dark colored and clayey ; 

 that from trap rock, brown and finely comminuted, but not argillaceous. 

 We cannot but believe, therefore, that the statement, that rocks have 

 Vol. XLiii, No. 1.— April-June, 1842. 25 



