PLUTONIC OR IGNEOUS ROCKS. 



651 



Serpentine often in alliance with diallage exhibits shades of light and dark green 

 with red varieties, has an unctuous feel, and a strongly argillaceous odour. It is found in 



rocks and beds in the Alpine districts of Europe, and 

 in- the promontory of the Lizard Point in Cornwall. 

 That kind of serpentine to which the term "noble" 

 has been applied, is found in contact with granite at 

 Portsoy in Aberdeen shire, where there is a mass 

 near the harbour upwards of five hundred yards wide. 

 The colours are, different shades of green and red, dis- 

 posed in clouds, veins, spots, and dots ; all these va- 

 rieties sometimes being present in hand specimens. 

 It is often called the Portsoy marble, and has been 

 exported to France to adorn the palace of Versailles. 

 The hill of Castellamonte, at the foot of the eastern 

 side of the Alps, not far from Turin, is a formation of 

 common serpentine, singularly veined with magnesite, 

 or the carbonate of magnesia. The cut shows a natural 

 section of this rock, from M. Brongniart. 



Porphyry the classical rock, which received its 

 name from its colour, Trop^upa, purple has a base of 

 compact felspar, with imbedded crystals of felspar. 

 It is one of the hardest of all rocks, and, when 

 polished, one of the most enduring : but the stone 

 used by the ancients for purposes of 

 adornment was rather crimson than 

 purple, and exhibits, in fact, almost every 

 variety of colour. The summit of Ben 

 Nevis, rising to the height of 4870 feet, 

 " the heaven-kissing hill," as some Gaelic 

 etymologists translate the title, is an 

 irregular four-sided prism of primitive 

 porphyry, of a dark grey or black colour, 

 superimposed upon a plateau of granite. The term porphyry is not employed now to 

 denote any particular kind of rock, but only a certain structure of rock, which will be 

 noticed under the next group. 



TRAPPEAN ROCKS. We derive the word trap from the Swedish Trappa, a stair, 

 applied to certain unstratified rocks, because of their frequent arrangement in the form of 

 a series of steps. The term is somewhat loosely employed ; but the rocks included in the 

 class it designates consist essentially of felspar, in combination with hornblende or augite, 

 or with both. It embraces several species, which are very generally diffused over the 

 globe, and furnish indisputable evidence of having been protruded from below in a state 

 of fusion, disturbing the sedimentary strata by their irruption, and becoming associated 

 with them in overlying masses, or injected into fissures produced by the disturbance, 

 forming dykes and interstratifications. They belong, apparently, to all ages, from the 

 time of the older granites to recent epochs ; and while in many instances they are similar 

 to the products of active volcanoes, in other cases they so much resemble the granite and 

 its kindred rocks, that, taken metaphorically, their generic title, as Mr. Bakewell observes, 

 becomes extremely appropriate, as these rocks offer a series of gradations, or steps, by 

 which the geologist may pass from the lava of Etna to the most ancient terrestrial forma- 

 tions. The igneous origin of the members of the trap family is very clearly shown by a 



A, diluvium or alluvium, composed of rolled pebbles and reddish 

 sand. B B, mass of pale green disintegrated serpentine, in which 

 veins of magnesite wind and osculate, a a a, veins of magnesite. 

 b, chalcedony in mammillated plates, in the midst of some of these 



