42 COMPOSITION OF SIMPLE MINERALS. 



Railway passes. Still, I liavo seen gooii crops of rye growing there, though the soil may 

 have derived a beneficial inlliience from the decomposition of the neighboring rocks com- 

 posed of hornblende and sienite. Here is also a peculiar vegetation : the Ilex canadensis^ 

 and some other herbaceous plants, are only found here, and this is the only place where 

 any thing like a pine grove has been planted by nature. For localities where serpentine 

 prevails, see the Report of the Second Geological District. 



In this connection, it will be proper to state the composition of basalt and greenstone, 

 although in New-York they do not form very extensive beds. 



Basalt. Greenstone. __ - 



Silica,... 4G.50 57.25 



Alumina, _-. 16.75 25.50 



Lime, 9.50 2.75 



Magnesia, 2.25 



Soda, 2.60 8.10 



Iron and manganese, 20.12 3.50 



Water, 2.00 3.00 



97.72 100.10 



The composition, however, of these varieties of rock is extremely variable, but all are 

 known to contain the alkalies and alkaline earths ; and it is owing to this fact that the 

 greenstone soils are remarkably fertile, so much so that they may often be employed to 

 increase the fertility of less favored ones. 



§ 4. Character of granitic soils. 



Returning once more to the consideration of granitic soils, 1 remark, that they are too 

 siliceous and porous when derived purely from granite. Position, however, alters their 

 character ; for where they lie upon sloping surfaces, sand predominates ; but in the 

 valleys, the fine alumine or clay of the felspar accumulates and forms an admixture of 

 clay and sand, which is more favorable to the support of grass and grain. On reviewing 

 the composition of the minerals wliidi enter as elements in rocks, we find that the most 

 abundant of tliem contain the proper proportions for a good soil. vSilex rarely forms less 

 than one-half; the remainder is made up of alumina (which is essential to the consistency 

 of the soil) , lime, potash, soda and iron, some containing more and some less of each 

 respectively, the alkalies being the most essential, and rendering a soil rich, as it is 

 termed, in proportion to their amount. In addition to the fact here stated, I may 

 observe that the tendency to decompose is also increased in proportion to the percentage 

 of the alkalies contained in the mineral : a rock of pun? quartz is acted upon very 

 slowly, while one in which felspar and mica exist crumbles rapidly. 



