Hypogene Rocks. Those rocks which are nether-formed, or which have not assumed their present form and structure at the surface, such as granite, gneiss, &c. The term, which includes both the plutonic and metamorphic rocks, is substituted for primary, because some members of both these classes, such as granite and gneiss, are posterior to many secondary or fossiliferous rocks. Etym., ὑπο, hypo, under, and γινομαι, ginomai, to be formed or produced.
Iceberg. Great masses of ice, often the size of hills, which float in the polar and adjacent seas. Etym., ice, and berg, German for hill.
Ichthyosaurus. A gigantic fossil marine reptile, allied in part of its structure to a fish. Etym., ιχθυς, ichthus, a fish, and σαυρα, saura, a lizard.
Igneous Rocks. All rocks, such as lava, trap, and granite, known or supposed to have been melted by volcanic heat.
Incandescent. White hot—having a more intense degree of heat than red heat.
Induction. A consequence, inference, or general principle drawn from a number of particular facts or phenomena. The inductive philosophy, says Mr. Whewell, has been rightly described as a science which ascends from particular facts to general principles, and then descends again from these general principles to particular applications.
Infusory Animalcules. Minute living creatures found in many infusions; and the term infusori has been given to all such animalcules, whether found in infusions or in stagnant water, vinegar, &c.
Inspissated. Thickened. Etym., spissus, thick.
Invertebrated Animals. Animals which are not furnished with a back-bone. For a further explanation, see "Vertebrated Animals."
Isothermal. Such zones or divisions of the land, ocean, or atmosphere, which have an equal degree of mean annual warmth, are said to be isothermal, from ισος, isos, equal, and θερμη, therme, heat.