We can't use both. The answer is to use the one with the smaller absolute value: With this definition of. A slightly different formal definition of the limit is used when x goes to +/- infinity, since we really cannot subtract x from infinity or vice versa-remember, infinity is a concept, not a number. Formally, the expression. Means that.
During the long interval of the Precambrian Era (which includes approximately 90% of geologic time and includes three Eons: the Hadean, the Archean and the Proterozoic) the only inhabitants of the Earth were simple microscopic organisms, many of them comparable in size and complexity to modern-day bacteria.
Life in the Tertiary Era On land, the age of the mammals was beginning. Rodents evolved, as well as gliding mammals and the early primates. Sharks and fish took over from marine reptiles. Towards the middle of this Era, the ancestors of elephants, horses, cattle, pigs, rhinos and deer appeared.eological Time Periods. The time periods are very important to a palaeontologist. The table below is a very simple Geological Time Line. Click on the time period you wish to invisigate.