Amygdaloid

Amygdaloid (Gr. amygdalē, 'an almond'), an igneous crystalline or, as the case may be, vitreous rock (lava), containing numerous cells, which owe their origin to the segregation and expansion of steam, with which all lavas are more or less charged at the time of their eruption. The cells vary in size from mere pores up to cavities several inches or even feet in diameter—these last, however, being exceptional, and when they do occur, quite sporadic. The cells are generally flattened or drawn out in the direction of flow of the lava, and are frequently filled with mineral matter (amygdules), subsequently introduced by infiltrating water. This is the origin of many of the agates and so-called 'Scotch pebbles' of jewellers. As cells and cellular structure occur in many different kinds of igneous rock, the term amygdaloid no longer denotes a rock-species, and has therefore fallen into disuse. It is now only employed in the adjectival form, amygdaloidal, as indicating a cellular or slaggy-like structure, in which the pores and cells are more or less filled up with mineral matter.