[Frost and fire : natural engines, tool-marks and chips : with sketches taken at home and abroad by a traveller] . urface, and it has probahly \\orked in tliesame direction from the beginning. And this mighty force is manifest within a weeks sail ofthe English coast; where its activity is often forgotten tillsome earthquake startles a sleeping to^vn. It seems impossible to visit Iceland and deny the im-portance of internal heat as a geological agent, or to revisitold haunts without recognizing traces of extinct volcanicaction everywhere. To ride over lava and gaze into craters is to gain fresh
[Frost and fire : natural engines, tool-marks and chips : with sketches taken at home and abroad by a traveller] . urface, and it has probahly \\orked in tliesame direction from the beginning. And this mighty force is manifest within a weeks sail ofthe English coast; where its activity is often forgotten tillsome earthquake startles a sleeping to^vn. It seems impossible to visit Iceland and deny the im-portance of internal heat as a geological agent, or to revisitold haunts without recognizing traces of extinct volcanicaction everywhere. To ride over lava and gaze into craters is to gain freshknowledge, and learn new tool marks. It is like visitingsome asteroid still warm from natures laboratory, or to readan early chapter in this worlds history. If any stones be preachers, surely these are eloquent. Modem geology, then, treats of sedimentary and of igne-ous rocks ; it recognizes the activity of two mechanical forces,which act in opposite directions, upward and downward,from and towards a centre, in radiating and converging lines ;and Iceland is ]ipculiarly fitted for studying the effects of Fio. 3. Nature and Iceland.—Badstua, near the Uxahver, IStil. /^^/^.^?o ,5/4 I V^^-^ZiJ^ CHAPTEK IV. Visible objects of all kinds are kuo\\Ta by their forms, bytheir outlines, and by their internal structure, as well as bycolour, weight, and chemical composition : and form has thisadvantage—it can be used as a test where others cannot beajDplied. Fossils, for example, are kno-svn to have been plants, shells,or bones, only because of their outward forms and internalstructure. Organized forms indicate previous orderly movements, andforces which produced them ; and the former activity of vitaland mechanical forces wliich now arrange the componentparts of living plants and animals is proved by form ; whencolour, weight, and chemical composition differ from those ofany livdng thing. A knowledge of form is veiy important to the student ofextinct life. So
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