362 „; General Strata. 



In the preceding Synopsis, a specimen of each general 

 stratum is represented by a wood cut figure. The order of 

 superposition and the direction of the strata may be perceiv- 

 ed at a single view. Several other characters of each stra- 

 tum are also exhibited by these figures. 



The original classes of Werner, (primitive, transition, and 

 secondary,) are retained ; because I cannot discover any ad- 

 vantages in the proposed alterations. I adopt Bakewell's 

 name for volcanic or basaltic rocks. And I prefer the old 

 general appellation, Detritus, to tertiary formation. For 

 boulders of ancient rocks cannot be considered as of the 

 tertiary formation, and the recently indurated Detritus, being 

 only a modification of it, in the form of puddingstone, &c., 

 does not require a particular name. 



Names under the Primitive Class.* 



1. Granite, is an aggregate of angular masses of quartz, 

 felspar, and mica. Subdivisions. — It is called crystalline, 

 (granite proper,) when the felspar and quartz present a crys- 

 talline, not a slaty, form. It is called slaty, (gneiss) when 

 the mica is so interposed in layers as to present a slaty form. 

 Varieties. — It is graphic, when the felspar is in a large pro- 

 portion, and the quartz is arranged in oblong masses, so as 

 to present an appearance resembling Chinese letters. It is 

 porphyritic, when spotted with cuboid blocks of felspar. 

 This variety is peculiar to the slaty division. 



2. Mica Slate, is an aggregate of grains of quartz and 

 scales of mica. Subdivisions. — Compact, when the slaty 

 laminae are so closely united, that it will present an uniform 

 smooth face when cut transversely. Fissile, when the la- 

 minae separate readily by a blow upon its surface. 



3. Hornblende RocK,t is an aggregate, not basaltic, con- 

 sisting wholly, or in part, of hornblende and felspar. Sub- 



* Every rock consists, essentially, of one, two, or three, of the following; 

 nine homogeneous minerals. These are called the geological alphabet; and 

 every student must procure, and familiarize himself with, a specimen of each, 

 before he commences the study of geology — quartz, felspar, mica, talc, horn- 

 blende, argillite, limestone, gypsum, chlorite. He should procure also a spe- 

 cimen of iron pyrites, hornstone, calc-spar, reddle-ore, bog-ore, glance-coal, 

 bituminous- coal. 



1 1 believe Maclure first applied this general name, to all the varieties of 

 primitive hornblende rock. 



