KINDS OF ROCKS. 79 



Lavas are varieties of all the above rocks from melaphyre to nephelinyte, and also 

 of trachytic and phonolytic rocks of the feldspathic series. 



Wacke is ah earthy rock made of basaltic earth partly compacted, or an earthy 

 altered dolerytic rock. 



Tufas and conglomerates of volcanic regions are noticed on page 66. 



(3.) Acidic and basic semes of igneous rocks. — As silica is the acid element in igneous 

 rocks, the other ingredients being basic, the kinds in which the silica exceeds 55 per 

 cent, are referred to an acidic series, and those with less to a basic series. The acidic 

 series includes granite, granul yte, trachyte, phonolyte, rhyolyte, and part of syenyte ; 

 and the basic, most hyposyenite and dioryte, with melaphyre, doleryte, peridotyte, 

 •amphigenyte, nephelinyte. The rocks of the feldspathic series belong with the former, 

 excepting those in which the feldspar is andesine, labradorite, or anorthite ; and those 

 of the hornblendic and pyroxene (or the iron-bearing) series with the latter, excepting 

 syenyte (containing quartz), and the varieties of hyposyenyte and dioryte, in which 

 orthoclase or albite is the chief feldspar. The average per-centage of silica in the 

 former is about 70, and in the latter about 51; and the oxygen ratio for the bases and 

 silica is in the former about 1 to 3, and in the latter 2 to 3. The two series pass into 

 one another. 



The principal recent works on Lithology are the following: Von Cotta's Treatise, 

 an English edition of which has been published in London; Blum's " Handbuch der 

 Lithologie," Erlangen, 1860; Senft's " Beschreibung der Felsarten, : ' Breslau, 1857; 

 Coquaxd's ; 'Traite" des Roches," Paris, 1857. 



II. CONDITION, STRUCTURE, AND ARRANGEMENT 

 OF ROCK-MASSES. 



The rock -masses of the globe, or terranes, as they are called, occur 

 under three conditions : (1) the stratified, (2) the unstratified, and 

 (3) the vein condition. Under each, there are peculiarities of struc- 

 ture and of ARRANGEMENT. 



1. STRATIFIED CONDITION. 

 Under this head the subjects for consideration are, — 1. The nature 

 of stratification ; 2. The structure of layers ; 3. The positions of 

 strata, — both their natural positions and their dislocations ; 4. The 

 general arrangement of strata, or their chronological order. 



1. Nature of Stratification. 



Stratified rocks are those which are made up of series of layers 



•or strata. The annexed sketch represents a section of the strata as 



exhibited along Genesee River, at 



° Fig. 60. 



the falls near Rochester. The whole A 



height of the section is 400 feet. 5d>\ 0S L 



At bottom there is a thick stratum ; ^^^- ^j fia^l sj 



of sandstone (1) ; next above it j. c ,,^ ^^^ ^^S ^^^^ 2 



lies a hard, gray layer (2), which 5Z/i Ji^^.^^^/'^ :y : ^ : ^3\ 



has been called the Gray Band. On 



this rests (3) a thick bed of greenish shale, a fragile, imperfectly slaty 



rock. Next (4) is a compact limestone, forming a widespread stratum 



