Ch. XXXV.] ORIGIN OF METAMORPHIC STRUCTURE. 743 



uine of the plutouic rocks, is in perfect accordance with the Huttonian 

 hypothesis of the intense heat to which this class of rocks has owed 

 its origin. 



In considering, then, the various data already enumerated, the 

 forms of stratification and lamination in metamorphic rocks, their 

 passage on the one hand into the fossiliferous, and on the other into 

 the plutonic formations, and the conversions which can be ascer- 

 tained to have occurred in the vicinity of granite, we may conclude 

 that gneiss and mica-schist may be nothing more than altered mica- 

 ceous and argillaceous sandstones, that granular quartz may have been 

 derived from siliceous sandstone, and compact quartz from the same 

 materials. Clay-slate may be altered shale, and granular marble may 

 have originated in the form of ordinary limestone, replete with shells 

 and corals, which have since been obliterated ; and, lastly, calcareous 

 sands and marls may have been changed into impure crystalline lime- 

 stones. 



"Hornblende-schist," says Dr. MacCulloch, "may at first have 

 been mere clay ; for clay or shale is found altered by trap into Lydian 

 stone, a substance differing from hornblende-schist almost solely in 

 compactness and uniformity of texture." * " In Shetland," remarks 

 the same author, " argillaceous-schist (or clay-slate), when in contact 

 with granite, is sometimes converted into hornblende-schist, the schist 

 becoming first siliceous, and ultimately, at the contact, hornblende- 

 schist." f 



The anthracite and plumbago associated with hypogene rocks may 

 have been coal ; for not only is coal converted iuto anthracite in the 

 vicinity of some trap dikes, but we have seen that a like change has 

 taken place generally even far from the contact of igneous rocks, in 

 the disturbed region of the Appalachians.}; At Worcester, in the 

 State of Massachusetts, 45 miles due west of Boston, a bed of plum- 

 bago and impure anthracite occurs, inter stratified with mica-schist. 

 It is about 2 feet in thickness, and has been made use of both as fuel 

 and in the manufacture of lead pencils. At the distance of 30 miles 

 from the plumbago, there occurs, on the borders of Rhode Island, an 

 impure anthracite in slates containing impressions of coal-plants of 

 the genera Pecopteris, JVeuropteris, Calamites, &c. This anthracite is 

 intermediate in character between that of Pennsylvania and the plum- 

 bago of Worcester, in which last the gaseous or volatile matter 

 (hydrogen, oxygen, and nitrogen) is to the carbon only in the pro- 

 portion of 3 per cent. After traversing the country in various direc- 

 tions, I came to the conclusion that the carboniferous shales or slates 

 with anthracite and plants, which in Rhode Island often pass into 

 mica-schist, have at Worcester assumed a perfectly crystalline and 



* Syst. of Geol., vol. i. p. 210 



f Ibid., p. 211. 



% See above, pp. 497, 503. 



