As I have said, it is possible to form an approximate idea of the relative age of the various strata by comparing them at different parts of the earth's surface. Geologists have long been agreed that there is a definite historical succession of the different strata. The various superimposed layers correspond to successive periods in the organic history of the earth, in which they were deposited in the form of mud at the bottom of the sea. The mud was gradually converted into stone. This was lifted out of the water owing to variations in the earth's surface, and formed the mountains. As a rule, four or five great divisions are distinguished in the organic history of the earth, corresponding to the larger and smaller groups of the sedimentary strata. The larger periods are then sub-divided into a series of smaller ones, which usually number from twelve to fifteen. The comparative thickness of the groups of strata enables us to make an approximate calculation of the relative length of these various periods of time. We cannot say, it is true, "In a century a stratum of a certain thickness (about two feet) is formed on the average; therefore, a layer 1000 feet thick must be 500,000 years old." Different strata of the same thickness may need very different periods for their formation. But from the thickness or size of the stratum we can draw some conclusion as to the RELATIVE length of the period. The first and oldest of the four or five chief divisions of the organic history of the earth is called the primordial, archaic, or archeozoic period. If we compute the total average thickness of the sedimentary strata at about 130,000 feet, this first period comprises 70,000 feet, or the greater part of the whole. For this and other reasons we may at once conclude that the corresponding primordial or archeolithic period must have been in itself much longer than the whole of the remaining periods together, from its close to the present day. It was probably much longer than the figures I have quoted (7 : 6) indicate--possibly 9 : 6. Of late years the thickness of the archaic rocks has been put at 90,000 feet. SYNOPSIS OF THE PALEONTOLOGICAL FORMATIONS, OR THE FOSSILIFEROUS STRATA OF THE CRUST. COLUMN 1 : Groups (V. down to I.). COLUMN 2 : Systems (XIV. down to I.). COLUMN 3 : Formations (38 down to 1). COLUMN 4 : Synonyms of Formations. V. Anthropolithic group, or anthropozoic (quaternary) group of strata : XIV. Recent (alluvium) : 38. Present : Upper alluvial. V. Anthropolithic group, or anthropozoic (quaternary) group of strata : XIV. Recent (alluvium) : 37. Recent : Lower alluvial. V. Anthropolithic group, or anthropozoic (quaternary) group of strata : XIII. Pleistocene (diluvium) : 36. Post-glacial : Upper diluvial. V. Anthropolithic group, or anthropozoic (quaternary) group of strata : XIII. Pleistocene (diluvium) : 35. Glacial : Lower diluvial.