580 



PLUTONIC ROCKS OF THE 



[Ch. XXXIV. 



Fig. 699. 



J unction of granite with Jurassic or Oolite strata in 

 the Alps, near Champoleon. 



mass the argillaceous beds are 

 hardened, the limestone is sac- 

 charoid, the gritz quartzose, 

 and in the midst of them is a 

 thin layer of an imperfect 

 granite. It is also an impor- 

 tant circumstance that near 

 the point of contact, both the 

 granite and the secondary 

 rocks become metalliferous, 

 and contain nests and small 

 veins of blende, galena, iron, 

 and copper pyrites. The stra- 

 tified rocks become harder and 

 more crystalline, but the gra- 

 nite, on the contrary, softer 

 and less perfectly crystallized 

 near the junction.* 

 Although the granite is incumbent in the above section (fig. 699), we 

 cannot assume that it overflowed the strata, for the disturbances of the 

 rocks are so great in this part of the Alps that they seldom retain the 

 position which they must originally have occupied. 



A considerable mass of syenite, in the Isle of Skye, is described by 

 Dr. MacCulloch as intersecting limestone and shale, which are of the 

 age of the lias."j" The limestone, which, at a greater distance from the 

 granite, contains shells, exhibits no traces of them near its junction, 

 where it has been converted into a pure crystalline marble. J 



At Predazzo, in the Tyrol, secondary strata, some of which are lime- 

 stones of the Oolite period, have been traversed and altered by plutonic 

 rocks, one portion of which is an augitic porphyry, which passes insen- 

 sibly into granite. The limestone is changed into granular marble, with 

 a band of serpentine at the junction.§ 



Carboniferous period. — The granite of Dartmoor, in Devonshire, was 

 formerly supposed to be one of the most ancient of the plutonic rocks, 

 but is now ascertained to be posterior in date to the culm-measures of 

 that county, which, from their position, and as containing true coal- 

 plants, are regarded by Professor Sedgwick and Sir R. Murchison as 

 members of the true carboniferous series. This granite, like the syeni- 

 tic granite of Christiania, has broken through the stratified formations 

 without much changing their strike. Hence, on the north-west side of 

 Dartmoor, the successive members of the culm-measures abut against the 

 granite, and become metamorphic as they approach. These strata are 



* Elie de Beaumont, sur les Montagnes de l'Oisans, &c. Me"m. de la Soc. 

 l'Hist. Nat. de Pai'is, torn. v. 



f See Murchison, Geol. Trans., 2d series, toI. ii., part ii., pp. 311 — 321. 

 % Western Islands, vol. i. p. 330, plate 18, figs. 3, 4. 

 g Yon Buch, Annales de Chimie, &c. 



