Ch. XXXV.] ORIGK^ OF METAMORPHIC STRUCTURE. 597 



in thickness, and has been made use of both as fuel, and in the raanu 

 facture of lead-pencils. At the distance of 30 miles from the plumbago, 

 there occurs, on the borders of Rhode Island, an impure anthracite in 

 slate^, containing impressions of coal-plants of the genera Pecopteris, Neu- 

 ropteris^ Calamites, &c. This anthracite is intermediate in character be- 

 tween that of Pennsylvania and the plumbago of Worcester, in which 

 last the gaseous or volatile matter (hydrogen, oxygen, and nitrogen) is to 

 the carbon only in the proportion of 3 per cent. After traversing the 

 country in various directions, I came to the conclusion that the carbonif- 

 erous shales or slates with anthracite and plants, which in Rhode Island 

 often pass into mica-schist, have at Worcester assumed a perfectly crys- 

 talline and metamorphic texture ; the anthracite having been nearly trans- 

 muted into that state of pure carbon which is called plumbago or 

 graphite.^ 



It has been remarked by M. Delesse that the minerals developed in 

 hypogene limestone vary according to the degree of metamorphism which 

 the rock has undergone. Thus, for example, where the structure is but 

 slightly crystalline, talc, chlorite, serpentine, andalusite, and kyanite are 

 commonly present ; where it is more highly crystallized, garnet, horn- 

 blende, Wollastonite, dipyre, Couzeranite, and some others appear ; and, 

 lastly, where the crystallization is complete, there are found, in addition 

 to many of the above minerals, felspar, especially those kinds which are 

 richest in alkah, together with mica. The same author observes that, as 

 calcareous deposits usually contain some aluminous clay, so we may nat- 

 urally expect to meet with silicates of alumina in crystalline limestone ; 

 such silicates, accordingly, are frequent, and occasionally even pure alumi- 

 na crystallized in the form of corundum.f 



Mr. Dana has suggested that the phosphoric acid of phosphate of lime- 

 and the fluor of fluor-spar, so often met with in crystaUine limestones, may 

 have been derived from the remains of mollusca and other animals ; also 

 that graphite (which is pure carbon in a crystalline form, with or without 

 admixture of alumina, lime, or iron) may have been derived from vegetable 

 remains imbedded in the original matrix. 



The total absence of any trace of fossils has inclined many geologists 

 to attribute the origin of the crystalline strata to a period antecedent to 

 the existence of organic beings. Admitting, they say, the obliteration, in 

 some cases,' of fossils by plutonic action, we might still expect that traces 

 of them would oftener occur in certain ancient systems of slate, in which, 

 as in Cumberland, some conglomerates occur. But in urging this argu- 

 ment, it seems to have been forgotten that there are stratified formations 

 of enormous thickness, and of various ages, and some of them very mod- 

 ern, all formed after the earth had become the abode of living creatures, 

 which are, nevertheless, in certain districts, entirely destitute of all ves- 

 tiges of organic bodies. In some, the traces of fossils may have been 

 eflfaced by water and acids, at many successive periods ; and it is clear, 



* See Ljell, Quart. Geol. Journ. vol. i. p. 199. 



f Delesse, Bulletin Soc. Geol. France, 2d serie, torn. 9, p. 126. 1851. 



