596 ORIGIN OF METAMORPHIC STRUCTURE. [Ch. XXXV 



tain masses of crystalline strata several miles in thickness. Now it has 

 been stated that the plutonic influence of the syenite of Norway has some- 

 times altered fossil iferous strata for a distance of a quarter of a mile, both 

 in the direction of their dip and of their strike. (See fig. '705, p. 593.) 

 This is undoubtedly an extreme case ; but is it not far more philosophical 

 to suppose that this influence may, under favorable circumstances, affect 

 denser masses, than to invent an entirely new cause to account for eff'ects 

 merely differing in quantity, and not in kind ? The metamorphic theory 

 does not require ns to aflSrm that some contiguous mass of granite has 

 been the altering power ; but merely that an action, existing in the in- 

 terior of the earth at an uaknown depth, whether thermal, hydro-thermal, 

 electrical, or other, analogous to that exerted near intruding masses of 

 granite, has, in the course of vast and indefinite periods, and when rising 

 perhaps from a large heated surface, reduced strata thousands of yards 

 thick to a state of semi-fusion, so that on cooling they have become crys- 

 talline, like gneiss. Granite may have been another result of the same 

 action in a higher state of intensity, by which a thorough fusion has been 

 produced ; and in this manner the passage from granite into gneiss may 

 be explained. 



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 forma- 

 tions, and the conversions which can be ascertained to have occurred in 

 the vicinity of granite, we may conclude that gneiss and mica-schist may 

 be nothing more than altered micaceous 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 obhter- 

 ated ; and, lastly, calcareous sands and marls may have been changed 

 into impure crystalHne limestones. 



" 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 some- 

 times converted into hornblende-schist, the schist becoming first sihceous, 

 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 into 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 rocl:s, in the 

 disturbed region of the Appalachians.]; At Worcester, in the state of 

 Massachusetts, 45 miles due west of Boston, a bed of plumbago and im- 

 pure anthracite occurs, interstratified with mica-schist. It is about 2 feet 



* Syst. of Geol vol. i. p. 210 f Ibid. p. 211. 



:f See above, p. 388, 394. 



