PLUTONIC OK IGNEOUS ROCKS. 



655 



presents that stair-like appearance which has originated the use of the word trap. Fig. 5. 

 is an example of this arrangement, from the neighbourhood of Kircudbright in Scotland, 



which shows the outcrop 

 of the porphyry on one side, 

 forming steps, and on the 

 other side, a projecting ver- 

 tical mass. Strictly speak- 

 ing, the former alone is 

 trap-porphyry. The por- 

 phyries are frequently asso- 

 ciated with primary strata, 

 and with granite. They 

 F,g. 5. a, Granwacke^ b> Porphyry. are j n general of more an- 



cient date than the basalts, which are commonly in conjunction with secondary rocks. 

 The tops of the Andes are enormous porphyritic piles, so arranged, according to Hum- 

 boldt, as to strike the eye of the traveller like immense castles lifted into the sky. 



Amyydaloids. This is also a term denoting a structure occasionally assumed by all the 

 members of the trap family. It indicates generally a base of fine-grained basalt, of a loose 

 earthy texture, in which pores or rounded cavities occur, which have been subsequently 

 filled up with fluid solutions of various mineral substances, and become consolidated into 

 nodules of calcareous spar or green earth, and more rarely of chalcedony and zeolite. 

 Hence such rocks often present the appearance of a paste studded with almonds, which 

 has originated their generic title of amygdaloids> from amygdala, an almond. The rock 

 familiarly called toadstone in Derbyshire is an amygdaloidal trap, consisting of a fine- 

 grained basalt paste of an iron colour, containing nodules of whitish calcareous spar and 

 green earth, varying in size from small spots to that of hazel-nuts. Where the 

 decomposition of these nodules has been effected, the toadstone assumes a vesicular and 

 lava-like character. 



There is no part of Great Britain, and but few places on the globe, which present so 

 great a variety of the igneous products noticed, within such narrow bounds, as the island 

 of Arran, off the coast of Scotland, lying in the bay formed by the peninsula of Cantire 

 and the Ayrshire shore. It is about twenty miles long by half that number broad. 

 The northern part of it consists of a great central mass of granite, which protrudes the 

 stratified formations of mica-slate and sandstone in a group of grand picturesque serrated 

 mountains, distinguished by their spiry forms, stupendous precipices, and general desti- 

 tution of vegetation. Goatfell, or, according to the Gaelic, Gaodh Bhein, the " Mountain 

 of Winds," the highest of them, rising to the height of 2865 feet, as estimated by a trigono- 

 metrical observation of Macculloch's, has an obtuse pyramid of granite for its summit, 

 consisting of large blocks, completely barren, with the exception of a few lichens. In 

 other parts of the island, the granite is often prismatic or cuboidal, forming immense 

 tabular masses, giving it the aspect of stratification In the southern district the red 

 sandstone is the predominating rock. This is penetrated by numerous trap veins, 

 consisting of dark porphyry with metallic diallage, black porphyritic greenstone, 

 earthy greenstone^ and claystone with nodules of green earth, all of which at several 

 points are closely aggregated ; and pitchstone has here one of its chief repositories, of 

 which Macctilloch has enumerated twenty-six varieties. 



VOLCANIC ROCKS. -^ We include in this class those ancient lavas and trachytic moun- 

 tains, the formation of which totfk place in geological epochs antecedent to the existing 

 arrangement of the earth, though of recent date, and clearly, from their nature, of volcanic 

 origin. The principal ingredients of trachytic rock are glassy felspar, with some horn- 



