GEOLOGY Sedimentary Rocks Quartzite, quartz schists and mica
... slates, which are succeeded by schistose conglomerates
containing pebbles of volcanic rocks probably derived trom
the earlier Cambrian lavas. Another unconformity precedes
the Ordovician System which commences with the West Coast
Range Conglomerate passing up into white or pinkish quartzites
which a ...
Ordovician
The Ordovician /ɔrdəˈvɪʃən/ is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 485.4 ± 1.9 and 443.8 ± 1.5 million years ago. It follows the Cambrian Period and is followed by the Silurian Period. The Ordovician, named after the Celtic tribe of the Ordovices, was defined by Charles Lapworth in 1879 to resolve a dispute between followers of Adam Sedgwick and Roderick Murchison, who were placing the same rock beds in northern Wales into the Cambrian and Silurian periods, respectively. Lapworth, recognizing that the fossil fauna in the disputed strata were different from those of either the Cambrian or the Silurian periods, realized that they should be placed in a period of their own. While recognition of the distinct Ordovician period was slow in the United Kingdom, other areas of the world accepted it quickly. It received international sanction in 1960, when it was adopted as an official period of the Paleozoic Era by the International Geological Congress.Life continued to flourish during the Ordovician as it did in the Cambrian, although the end of the period was marked by a significant mass extinction. Invertebrates, namely molluscs and arthropods, dominated the oceans. Fish, the world's first true vertebrates, continued to evolve, and those with jaws may have first appeared late in the period. Life had yet to diversify on land.