Ch. XXX VII.] MINERAL CHARACTER OF HYPOGEXE ROCKS. 703 



that the calcareous formation in question was of Lower Silurian age. 

 This was one of the most important steps made of late years in the 

 progress of British Geology, for it led to a very unexpected conclu- 

 sion, namely, that all the Scotch crystalline strata to the eastward, 

 once called primitive, which overlie the limestone and quartzite in 

 question, are referable to some part of the Silurian series. The 

 most abundant and best preserved shells of the limestone are those 

 obtained from Durness and Assynt. They comprise, among others, 

 three or four species of Orthoceras, also the genera Cyrtoceras and 

 Lituites, two species of Murchisonia, a Pleurotomaria, a species 

 of Maclurea, one of Euomphalus, and an Orthis. Several of the 

 species are believed by Mr. Salter to be identical with Lower Silurian 

 fossils of Canada and the United States. The mere occurrence of 

 Cephalopoda in such numbers is strongly against the supposition of 

 their being Cambrian, and the large siphuncles of some of the Ortho- 

 cerata point distinctly to a Lower Silurian date, for this division 

 of the genus, both in Europe and North America (see p. 565), is 

 eminently characteristic of the inferior members of the Silurian 

 system (see above, p. 565). To the fossiliferous rock above men- 

 tioned, with its accompanying quartzites, succeed in conformable 

 stratification a dense series of gneiss, mica-schists, and clay-slates, 

 this younger gneiss being very different in mineral character from the 

 fundamental gneiss before mentioned. There can be no question 

 that these crystalline formations, which are similar to those of the 

 Central and Southern Highlands, comprising the metamorphic rocks 

 of Aberdeenshire, Perthshire, and Forfarshire, for example, are 

 altered Silurian strata ; * the inferences of Sir E. Murchison on this 

 subject having been confirmed by the subsequent observations of 

 three able geologists, Messrs. Ramsay, Harkness, and Geikie. The 

 newest of the series is a clay-slate, on which, along the southern 

 borders of the Grampians, the Lower Old Red, containing Cepha- 

 laspis Lyelli, Pterygoius Anglicus, and Parka decipiens, rests uncon- 

 formably. 



In Anglesea, as was before remarked, the metamorphism of the 

 schists, according to the observations of Professor Ramsay, took 

 place during the Lower Silurian period. Coupling these conclusions 

 with the fact that a hypogene texture has been superinduced in the 

 Alps on Middle Eocene deposits (see p. 746), we cannot doubt that, 

 hereafter, geologists will succeed in detecting crystalline schists of 

 almost every age in the chronological series, although the quantity of 

 metamorphic rocks visible at the surface must, for reasons above ex- 

 plained, diminish rapidly in proportion as the monuments of newer 

 eras are investigated. 



Order of Succession in Metamorphic Pocks. — There is no universal 

 and invariable order of superposition in metamorphic rocks, although 



* Quart. Geol. Journ., vol. xv. p. 353, 1859. Siluria, 3d ed., Appendix, p. 553. 



