42(12 MIlSnERAL PHYSIOLOGY. 



We have thus distinguislied in the earth's crust two 

 classes of rock-masses ; the first formed after the man- 

 ner of clays, sands, marls, and peats, or like rock-salt and 

 gypsum from the drying-up of salt-water basins, as in 

 great deserts. These indigenous rocks may be either 

 crystalline or uncrystalline. The second or exotic class, 

 for the most part crystalline in character, consists of 

 fused or softened matters which have come up from be- 

 low, and have either occupied rifts or fissures in the pre- 

 viously formed rocks, or have overspread them. 



There is, however, a third class of rock-masses dis- 

 tinct from both of the preceding in their origin and mode 

 of formation. For the most part crystalline in character, 

 they occupy, like the exotic rocks, fissures or other cavi- 

 ties in previously formed rocks, but give evidence that 

 instead of having been introduced at one operation in a 

 softened form into their ijresent places, they have slowly 

 grow T n there by accretion, for which reason they have 

 been called endogenous rocks. These masses, in their 

 mode of origin are like the solid deposits from certain 

 mineral springs, or the incrustations in steam -boilers, and 

 though small in extent as compared with the indigenous 

 or exotic rocks, (in both of which they may occur), are 

 important alike from their mineral composition, and 

 from the light which they throw on terrestrial processes. 

 To these endogenous masses belong the so-called granitic 

 veins made up of the elements of ordinary granite, such 

 as feldspars, micas and quartz, but often containing tour- 

 maline, topaz, beryl, zircon and other gems, as well as 

 ores of tin, and some other metals. To this same class 

 also belong the mineral lodes from which are derived the 

 greater part of our gold, silver, copper and lead. Endo- 

 genous masses may be described as for the most part as 

 either granitic, quartzose or calcareous in character, but 

 as including at the same time, in a more or less concen- 

 trated form, many of the rarer chemical elements of the 

 earth's crust. While the silica, alumina, lime, magnesia, 

 alkalies and iron, which make up the chief part of the 

 solid rocks, are everywhere distributed with impartial 

 abundance, there are other elements, often of great val- 

 ue to man, which are present in the great bulk of the 

 earth in such insignificant amounts that their proportion 

 seems infinitely small. Aided by the spectroscope and 



