PART 1. CHAPTER VII. 



97 



Sconce Pumice 



Lava. 



of trap. It derives its name from the Greek word amygdala, 

 an almond. The origin of this structure cannot be doubted, for 

 we may trace the process of its formation in modern lavas. 

 Small pores or cells are caused by bubbles of steam and gas 

 confined in the melted matter. After or during consolidation 

 these empty spaces are gradually filled up by matter separating 

 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 scoriaceous lavas, or the slags of furnaces. 



Fig. 88. The annexed figure re- 



presents a fragment of 

 stone taken from the upper 

 part of a sheet of basaltic 

 lava in Auvergne. One 

 half is scoriaceous, the 

 pores being altogether 

 empty, the other part is 

 amygdaloidal, the pores or 

 cells being mostly filled 

 up with carbonate of lime, 

 forming white kernels. 



Scoriceand Pumice may 

 next be mentioned as po- 

 rous rocks, produced by 



Swriaceous lava in part converted into an amyg- flip n^finn nf rrncpq rm ma. 

 datad-Montasne de la Veille, Department of me . * C 



Puy de Dome, France. tenals melted by volcanic 



heat. Rcoricz are usually of a reddish brown and black colour, 

 and are the cinders and slags of basaltic or augitic lavas. Pumice 

 is a light, spongy, fibrous substance, produced by the action of 

 gases on trachytic and other lavas ; the relation, however, of its 

 origin to the composition of lava is not yet well understood. Von 

 Buch says it does not occur where only Labrador-felspar is 

 present. 



Lava. This term has a somewhat vague m signification, hav- 

 ing been applied to all melted matter observed to flow in 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 scoria- 

 ceous rock very frequently occurs, formed by the first thin sheet 

 i 



