LECTURE XII. 



Composition of the granitic rocks and of their constituent minerals— Cause ami made of 

 liieir degradation— Soils derived from them— Superficial accumulations— Their influence 

 upon the character of tlie soils — Organic constituents, ultimate chemical constitution, and 

 jiJiysic-al properties of soils. 



It has been stated in the preceding Lecture, (§ 6, p. 237), that the rocks 

 which present themselves at the surface of the earth arc of two kinds, 

 distinguished by the terms stratified and unstratified. The former 

 crumble away, in general, more rapidly than the latter, and form a va- 

 riety of soils of which the agricultural characters and capabilities have 

 been shortly explained. The unstratified or crystalline rocks form soils 

 of so peculiar a character and possessing agricultural capabilities in 

 general so different from those of the stratified rocks which occur in the 

 same neighbourhood, and they, besides, cover so^arge and hitherto so 

 unfruitful an area in our island, as to entitle them to a separate and 

 somewhat detailed consideration. 



§ 1. Composition of the Granitic Rocks. 



The name of Granite is given by mineralogists to a rock consisting of 

 a mixture more or less intimate of three simple minerals — Quartz, Mica, 

 and Felspar. When Mica is wanting, and Hornblende occurs in its 

 stead, the rock is distinguished by the name of Syenite. This mineral- 

 ogical distinction is often neglected by the geologist, who describes large 

 tracts of country as covered by granitic rocks, though there may be 

 many hills or mountains of syenite. In a geological sense, the distinc- 

 tion is often of little consequence; in relation to agriculture, however, 

 the distinction between a granite and a syenite is of considerable im- 

 portance. 



The minerals of which these rocks consist are mixed together in very 

 variable proportions. Sometimes the quartz predominates, so as to con- 

 stitute two-thirds or three-fourths of the whole rock, sometimes both 

 mica and quartz are present in such small quantity as to form what is 

 then called a felspar rock. The mica rarely exceeds one-sixth of the 

 whole, while the hornblende of the syenites sometimes forms nearly 

 one half of the entire rock. These differences also are often overlooked 

 by the geologist — though they necessarily produce important differences 

 in the composition and agricultural characters of the soils derived from 

 the crystalline rocks. 



A few other minerals occur occasionally among the granitic rocks, in 

 sufficient quantity to affect the composition of the soils to which they 

 give rise. Among these, the different varieties of tourmaline are in 

 many places abundant. Thus the schorl rock of Cornwall consists of 

 quartz and schorl (a variety of tourmaline), while crystals of schorl 

 are «o frequently found in the granites of Devon, Cornwall, and the 



