of birds, made when these rocks were in the state of soft mud. These layers form what is termed the crust of the earth, and are altogether several miles in thickness. Yet not one of these layers gives us the element of time. They announce to us successive generations of animals and plants; but they do not tell us how long these generations lived. We have every reason to believe that the condition of the world was very different then, from what it is now; not only as regards its temperature, of which we have many proofs that it was much higher than at present; but likewise in regard to the density of the atmosphere and the distribution of water on the surface of the globe. All these conditions indicate that both animal and vegetable life were then far different from what they are now, as the fossil remains of those animals and plants abundantly and unquestionably prove. But in all this we have no means of determining the duration of those species. The various species of plants and animals may have flourished during a period of a thousand, a million, or a thousand million years for all we know. {43} [ILLUSTRATION: SECTION OF THE CRUST OF THE EARTH] RECENT DEPOSITS--Showing remains of animals still existing. QUATERNARY--Showing remains of some extinct and some living species. First appearance of deciduous trees, (those that drop their leaves in autumn) showing alternations of heat and cold. Remains of vertebrates (animals having back bones) of the more perfect species. Mammals, (milk-giving animals); carnivora, (flesh-eating animals), monkeys, etc. First appearance of birds and land monsters, megatheriums, mammoths, etc. CRETACIOUS ROCKS--Age of reptiles and amphibious monsters, as iguanodons, saurians and crocodiles. CARBONIFEROUS ROCKS--Age of coal formation; first indications of reptiles, numerous amphibious animals--supposed uniform climate. SUB-CARBONIFEROUS--First traces of amphibious animals. First traces of cone-bearing trees, pines, ferns, etc. First fishes; first vertebrates; numerous shell fish; limestone strata. Chalk measures; sandstone strata, SILURIAN ROCKS--Few traces of life; old red sandstone. HURONIAN ROCKS--No traces of life; gneiss, quartz, felspar, mica. LAURENTIAN Rocks--Granite.