Ch. XXVffl.] AMYGDALOID— LAVA. 001 



be greenstone, basalt, or pitchstone, the rock may be denominated 

 greenstone-porphyry, pitch-stone porphyry, and so forth. The old 

 classical type of this form of rock is the red porphyry of Egypt, or the 

 well known " Rosso antico." It consists, according to Delesse, of a 

 red felspathic base in which are disseminated rose-colored crystals of 

 the felspar called oligoclase, with some plates of blackish hornblende 

 and grains of oxidized iron-ore (fer oligiste). Red quaftziferous por- 

 phyry is a much more siliceous rock, containing about 70 or 80 per 

 cent, of silex, while that of Egypt has only 62 per cent. 



Amygdaloid. — This is also another form of igneous rock, admitting 

 of every variety of composition. It comprehends any rock in which 

 round or almond-shaped nodules of some mineral, such as agate, chal- 

 cedony, calcareous spar, or zeolite, are scattered through a base of 

 wacke, basalt, greenstone, or other kind of trap. It derives its name 

 from the Greek word amygdala, an almond. The origin of this struc- 

 ture cannot be doubted, for we may trace the process of its forma 

 tion in modern lavas. Small pores or cells are caused by bubbles of 

 steam and gas confined in the melted matter. After or during con- 

 solidation, these empty spaces are gradually filled up by matter sepa- 

 rating from the mass, or infiltered by water permeating the rock. As 

 these bubbles have been sometimes lengthened by the flow of the 

 lava before it finally Cooled, the contents of such cavities have the 

 form of almonds. In some of the amygdaloidal traps of Scotland, 

 where the nodules have decomposed, the empty cells are seen to have 

 a glazed or vitreous coating, and in this respect exactly resemble sco- 

 riaceous lavas, or the slags of furnaces. 



The foregoing figure (676) represents a fragment of stone taken 

 from the upper part of a sheet of basaltic lava in Auvergne. One- 

 half is scoriaceous, the pores being perfectly empty ;, the other part 

 is amygdaloidal, the pores or cells being mostly filled up with carbon- 

 ate of lime, forming white kernels. 



Lava. — This term has a somewhat vague signification, having been 

 applied to all melted matter observed to fiowin streams from volcanic 

 vents. When this matter consolidates in the open air, the upper part 

 is usually scoriaceous, and the mass becomes more and more stony as 

 we descend, or in proportion as it has consolidated more slowly and 

 under greater pressure. At the bottom, however, of a stream of lava, 

 a small portion of scoriaceous rock very frequently occurs, formed by 

 the first thin sheet of liquid matter, which often precedes the main 

 current, or in consequence of the contact with water in or upon the 

 damp soil. 



The more compact lavas are often porphyritic, but even the scoria- 

 ceous part sometimes contains imperfect crystals, which have been 

 derived from some older rocks, in which the crystals preexisted, but 

 were not melted, as being more infusible in their nature. 



Although melted matter rising in a crater, and even that which 

 enters a rent on the side of a crater, is called lava, yet this term be- 



