Cambrian period

The Cambrian is a major division of the  geologic timescale that begins about 542 ± 1.0 Ma (million years ago) at the end of the Proterozoic eon and ended about 488.3 ± 1.7 Ma with the beginning of the Ordovician period (ICS, 2004). It is the first period of the Paleozoic era of the Phanerozoic eon. The Cambrian is named for Cambria, the classical name for Wales, the area where rocks from this time period were first studied.
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Noun
1. from 544 million to about 500 million years ago; marine invertebrates
(synonym) Cambrian
(hypernym) period, geological period
(part-holonym) Paleozoic, Paleozoic era

570-505 Million Years Ago
This starting period of the Paleozoic era was characterized by a diverse eruption of life in the seas. Life had not yet evolved on land. The highest forms of life existing at this time were the trilobites, which reached lengths up to two feet. Green and red algae started to appear as well as sponges, gastropods, and segmented worms.
Cambrian 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 "