KINDS OF ROCKS. 79 



Eucryte is a doleryte-like rock consisting chiefly of anorthite and augite. Occurs 

 compact and as a lava. Teschenite is bluish-green, and chiefly consists of anorthite, 

 hornblende, and augite, the hornblende sometimes in large black prisms; also contains 

 analcite ; from Teschen, Austria. 



8. Amphigenyte (Leucitophyre). — Contains augite, like doleryte, but leucite 

 (called sometimes amphigene) replaces the feldspar. Dark gray, fine-grained, and 

 more or less cellular to scoriaceous. Sp. gr. 2-7-2*9. The leucite is disseminated in 

 grains or in 24-faced ciystals ; constitutes the lavas of Vesuvius and some other re- 

 gions. 



9. Nephelinyte (Nepheline-doleryte). — Contains augite, like doleryte, but nephe- 

 lite replaces the feldspar, or the larger part of it. Crystalline-granular; ash-gray to 

 dark gray. The nephelite is partly in distinct crystals. 



10. Tachylyte. Hyalomelak. — Blackish glass, or pitchstone, made in connection 

 with augitic igneous rocks or lavas; afford on analysis 49 to 55 five per cent, of silica. 



3. Contain Chrysolite, and little or no Feldspar. 



1. Dunyte. — Yellowish-green ; granular. Consists of chrysolite, with some chrom- 

 ite. From Mount Dun, New Zealand. 



2. Lherzolyte. — Greenish-gray; crystalline granular. Consists of chrysolite, 

 enstatite, whitish pyroxene, with chrome-spinel and sometimes garnet. From Lake 

 Lherz, etc. Is it metamorphic? 



3. Picryte. — Blackish-green to brownish-red; crystalline-granular. Consists of 

 chrysolite, with augite sometimes in crystals. Graduates into chrysolitic doleryte. 



Limburgite is a semi-glassy rock of nearly similar composition. Affords on analysis 

 43 per cent, of silica. 



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 

 the falls near Rochester. The whole 

 height of the section is 400 feet. „«,. 

 At bottom there is a thick stratum 

 of sandstone (1) ; next above it 



lies a hard, gray layer (2), which 5Z/| ^Jzk~^~ ^^—zT^y^ - - "S^qt 

 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 





Fig. 60. 



A. 



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