612 AGE OF METAMORPHIC ROCKS [Ch. XXXVII 



schist, and another not far distant, where the same bed remains unaltered 

 and fossiliferous ; but when we have to compare two portions of a moun- 

 tain chain — the one metamorphic, and the other unaltered — all the labor 

 and skill of the most practised observers are required, and may sometimes 

 be at fault. I shall mention one or two examples of alteration on a grand 

 scale, in order to explain to the student the kind of reasoning by which 

 we are led to infer that dense masses of fossiliferous strata have been con- 

 verted into crystalline Yock. 



Northern Appenines — Carrara. — The celebrated marble of Carrara, 

 used in sculpture, was once regarded as a type of primitive limestone. 

 It abounds in the mountains of Massa Carrara, or the " Apuan Alps," 

 as they have been called, the highest peaks of which are nearly 6000 

 feet high. Its great antiquity was inferred from its mineral texture, 

 from the absence of fossils, and its passage downward into talc-schist 

 and garnetiferous mica-schist ; these rocks again graduating downwards 

 into gneiss, which is penetrated, at Forno, by granite veins. Now the 

 researches of MM. Savi, Boue, Pareto, Guidoni, De la Beche, Hoffmann, 

 and Pilla, have demonstrated that this marble, once supposed to be 

 formed before the existence of organic beings, is, in fact, an altered 

 limestone of the Oolitic period, and the underlying crystalline schists 

 are secondary sandstones and shales, modified by plutonic action. In 

 order to establish these conclusions it was first pointed out, that the cal- 

 careous rocks bordering the Gulf of Spezia, and abounding in Oolitic 

 fossils, assume a texture like that of Carrara marble, in proportion as 

 they are more and more invaded by certain trappean and plutonic rocks, 

 such as diorite, euphotide, serpentine, and granite, occurring in the same 

 country. 



It was then observed that, in places where the secondary formations 

 are unaltered, the uppermost consist of common Apennine limestone 

 with nodules of flint, below which are shales, and at the base of all, ar- 

 gillaceous and siliceous sandstones. In the limestone, fossils are frequent, 

 but very rare in the underlying shale and sandstone. Then a gradation 

 was traced laterally from those rocks into another and corresponding 

 series, which is completely metamorphic ; for at the top of this we find 

 a white granular marble, wholly devoid of fossils, and almost without 

 stratification, in which there are no nodules of flint, but in its place 

 siliceous matter disseminated through the mass in the form of prisms of 

 quartz. Below this, and in place of the shales, are talc-schists, jasper, 

 and hornstone ; and at the bottom, instead of the siliceous and argilla- 

 ceous sandstones, are quartzite and gneiss.* Had these secondary strata 

 of the Apennines undergone universally as great an amount of transmu- 

 tation, it would have been impossible to form a conjecture respecting 

 their true age ; and then, according to the method of classification 

 adopted by the earlier geologists, they would have ranked as primary 



* See notices of Savi, Hoffmann, and others, referred to by Boue, Bull, de la 

 Soc Geol. de France, torn. v. p. 317; and torn. iii. p. xliv; also Pilla, cited by 

 Murcbison, Quart. Geol. Journ. voL v. p. 266. 



