PART 11. CHAPTER XXV. 



295 



Age of the Metamorphic Rocks of the Northern Apennines. 



of science, we should succeed in ascertaining the twofold chro- 

 nological relations of the metamorphic formations, it might be 

 useful to adopt a twofold terminology. We might call the strata 

 above alluded to Liassic-Eocene, or Liassic-Cretaceous ; the first 

 term referring to the era of deposition, the second to that of 

 crystallization. According to this method, the chlorite-schist, 

 mica-schist, and gneiss of the Malvern Hills, would belong to 

 the Silurian Old Red sandstone period, because they are Silurian 

 strata altered into metamorphic rocks during the deposition of 

 the Old Red sandstone. (See p. 288.) 



We have seen, when discussing the ages of the plutonic rocks, 

 that examples occur of various primary, secondary, and tertiary 

 deposits converted into metamorphic strata, near their contact 

 with granite. There can be no doubt in these cases that strata, 

 once composed of mud, sand, and gravel, or of clay, marl, and 

 shelly limestone, have for the distance of several yards, and in 

 some instances several hundred feet, been turned into gneiss, 

 mica-schist, hornblende-schist, chlorite-schist, quartz rock, sta- 

 tuary marble, and the rest. (See Chapters 10. and 11.) 



But when the metamorphic action has operated on a grander 

 scale, it tends entirely to destroy all monuments of the date of 

 its development. It may be easy to prove the identity of two 

 different parts of the same stratum ; one, where the rock has 

 been in contact with a volcanic or plutonic mass, and has been 

 changed into marble or hornblende-schist, and another not far 

 distant, where the same bed remains unaltered and fossiliferous ; 

 but when we have to compare two portions of a mountain chain — 

 the one metamorphic, and the other unaltered — all the labour and 

 skill of the most practised observers are required. I shall men- 

 tion 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 

 converted into crystalline rocks. 



Northern Appenines. — Carrara. — The celebrated marble of 

 Carrara, used in sculpture, was once regarded as a type of primi- 

 tive 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 downwards 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, and especially Hoffman, have demonstrated that this 

 marble, once supposed to be formed before the existence of 



