THE STORY TOLD BY FOSSIL PLANTS
to increase in size for many years and carry aloft an ever larger canopy of leaves, and the development of seeds, which are a much more efficient means of reproduction than the simple single-celled spores of the lower plants.
Geologists construct their history in much the same way as any other historians, but instead of dealing with written documents or the handiwork of man they deal with the series of rocks that make up the crust of the earth, and especially with the fossils preserved in the rocks—the remains of the plants and animals that were alive when the rocks were being deposited as mud or sand. (See geological time table on page 160.)
You might suppose that this record of the past would be so scanty and broken that we could not read it. It is, indeed, far from complete, but when we remember that even the formation of a single bed of sandstone or of clay consumed a long time we can see that innumerable plants and animals might have been covered up by accumulating sediments and so well preserved that we could use them in our study of the earth’s history.
The earliest chapter of the world’s organic history we call the time of ancient life, or the Palaeozoic era, which is saying the same thing in Greek. This Palaeozoic era we divide into periods, each marked by distinctive types of fossils. The names of the periods that make up the Palaeozoic era, given in order from the oldest to the youngest, are Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian. Some of these names are geographical, each derived from the name of some place where rocks of that age are exposed. Cambrian is from Cambria, the Latin name of Wales; Silurian is from the name of a tribe—the Silures—which in Roman times inhabited that part of Britain where the Silurian rocks
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