Some occurs as crystals in igneous rocks, and some in vein deposits.
igneous rocks are formed by the cooling of molten rock material.
Mineral deposits formed in older geologic periods have in most cases been deeply covered by later sediments and igneous rocks.
These are characterized by igneous rocks of andesitic nature.
Let it be observed, in the first place, that there is no hard and fast line between the varieties of igneous and volcanic rocks.
I did not see any eruption of igneous rock on any part of the day's journey.
A subordinate amount may come directly from igneous emanations or from destruction of certain hydrous minerals.
To the southward, for fifty miles at least, nothing but igneous rocks can be seen.
In general, graphite is primarily concentrated both by igneous processes in dikes, and by sedimentary processes in beds.
Occurs also as crystals in some limestones and igneous rocks.
1660s, from Latin igneus "of fire, fiery," from ignis "fire," from PIE *egni- "fire" (cf. Sanskrit agnih "fire, sacrificial fire," Old Church Slavonic ogni, Lithuanian ugnis "fire").