Have we any knowledge regarding the beginning of life on our planet? Our answer to this question must be decidedly in the negative. We can, however, be very positive in regard to two important matters concerning life in early geological time, namely, that plants must have existed before animals, and that the very oldest known (Archeozoic) rocks of the earth contain vestiges of organisms. We may be sure that plants preceded animals because animal life ultimately depends upon plants for its food supply or, in other words, all animals could never have been carnivorous. Now, if we can prove that organisms existed during Archeozoic time, it is evident that plants at least must have lived in that oldest known era of earth history. That living things did then exist is proved by the common occurrence of graphite, a crystallized form of carbon, in the oldest known of the Archeozoic rocks. The facts that flakes of graphite are abundantly scattered through many layers of strata of Archeozoic Age, and that adjacent layers of strata contain such varying amounts of graphite, render it practically certain that such graphite represents the carbon of organisms. Graphite existing under such conditions could not be of igneous origin. Carbonaceous or bituminous strata, so called because they contain more or less decomposed organic matter, would, when crystallized under conditions of metamorphism, yield graphite-bearing rocks exactly like those of Archeozoic Age, and there is every reason to believe that this was their origin. But, since only graphite (carbon) of the Archeozoic organisms remains, the rest having disappeared through chemical change or decomposition, it is impossible to say whether much or all of it represents original plants or animals. In any case we can be very sure about the existence of plants (probably very simple or primitive types) in Archeozoic time, but the presence of any form of animal life has not been proved. In the next, or Proterozoic era, some plants and animals of definite types are known to have existed and, from here on in the present chapter, it is our purpose to consider the salient points in the geological history of plants, taking up the main types in the regular order of their appearance from the remote Proterozoic days to the present. The very oldest known definitely determinable fossils of any kind are the more or less rounded masses of crudely concentric layers of carbonate of lime from one to fifteen inches in diameter found in middle Proterozoic limestone of western Ontario, Canada. Similar forms are abundant in late Proterozoic strata of Montana. They occur in large numbers as layers or reefs, in many cases repeating themselves through hundreds or even thousands of feet of strata. Careful studies have shown that these forms are the limey secretions of some of the very simplest types of plants, that is thallophytes (e.g., seaweeds), which lived in water.