General Strata. 363 



divisions. — Granitic, when it presents the appearance of 

 crystalline granite, with hornblende substituted for mica. 

 Slaty, when of a rifty or tabular structure. Varieties. — 

 Gneisseoid, when it resembles slaty granite (gneiss) with 

 scales of hornblende substituted for mica. Greenstone, when 

 of a pretty uniform green color, and containing but a small 

 proportion of felspar, generally of a slaty structure. Por- 

 phyritic, when spotted with cuboid blocks of felspar. Sien- 

 itic, when speckled with small irregular masses of felspar. 



4. Talcose Slate, is an aggregate of grains of quartz and 

 scales of mica and talc* Subdivisions. — Compact, having 

 the lamina? so closely united that a transverse section may 

 be wrought into a smooth face. When the quartzose parti- 

 cles are very minute and in a large portion, it is manufactur- 

 ed into scythe-whetstones, called Quinnebog stones. Fis- 

 sile, when the laminae separate readily by a blow upon the 

 surface. Varieties. — Chloritic, when colored green by chlo- 

 rite. In some localities the chlorite seems to form beds ; or 

 rather the rock passes into an aggregate consisting of quartz, 

 mica, talc, and a large proportion of chlorite. Vast beds of 

 pure chlorite are embraced in this rock on Deerfield river, in 

 Florida, Mass. 



5. Granular Quartz, consists of grains of quartz united 

 without cement. Subdivisions. — Compact, when it consists 

 of fine grains, so as to appear almost homogeneous ; gener- 

 ally in large rhomboidal blocks. Sandy, when the grains 

 are so slightly attached as to be somewhat friable. Varie- 

 ties. — Translucent, when it is so compact and homogeneous 

 as to transmit light. Yellow, when slightly tinged with iron, 

 (probably a carbonate.) Ferruginous, when an aggregate 

 of minute crystals, strongly colored yellow or red with the 

 carbonate or peroxyd of iron. There is a remarkable local- 

 ity two miles north of Bennington village, in Vermont. Large 

 masses may be found consisting of six-sided crystals, with 

 six-sided pyramids on both ends. 



6. Granular Limestone, consists of glimmering grains 

 of carbonate of lime united without cement. Subdivisions. — 



* That a small proportion of talc scales should serve to distinguish this rock 

 from mica-slate, would scarcely satisfy a mere cabinet student. But the tra- 

 velling geologist will acknowledge its importance. See Taghconnuc and Sad- 

 die mountains, and the same range along the west side of the Green mountains 

 to Canada. 



