Ordovician period

The Ordovician period is the second of the six (seven in North America) periods of the Paleozoic era, and covers the time roughly between 490 to 440 million years ago. It follows the Cambrian period and is followed by the Silurian period. The Ordovician, named after the Welsh 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.
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Noun
1. from 500 million to 425 million years ago; conodonts and ostracods and algae and seaweeds
(synonym) Ordovician
(hypernym) period, geological period
(part-holonym) Paleozoic, Paleozoic era

505-438 Million Years Ago
The first vertebrate fish have been found to have existed during the early Ordovician period. Graptolites and corals also flourished.
Ordovician Sea 
see list of time periods

Geological Eras When H. P. Blavatsky was writing about the age of the earth in The Secret Doctrine she compared the teachings of the scientists of that time and found nothing but confusion and uncertainty as to geological figures. However, Professor Lefevre in his Philosophy adopted an original method of interpreting the data available. Instead of trying to reach exact figures in regard to the length of the entire fossil-bearing period of sedimentation from the Laurentian period to the present day, or of its subdivisions, he worked out the relative durations of the sedimentary deposits. With this for a background the actual duration of the eras and periods could easily be calculated when reliable evidence was found. Lefevre's studies were based on the erosion of rocks and the deposition of sediments, and his conclusions have stood with little modification till now. H. P. Blavatsky noticed that his estimates of the relative duration of the geological ages agreed fairly well with the 'esoteric' information in her possession, and so by adapting her knowledge of the real figures to Lefevre's proportional scale she constructed a time table which, she says, approximates the truth "in almost every particular." Her total of "320,000,000 years of sedimentation" is much less than that of modern geologists, even though she includes the Laurentian period in her table, which they omit. Her "Esoteric" table (Sd 2:710) is as follows:
to be continue "Geological Eras2 "