The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416. 0 ± 2. 8 Mya ( million years ago ), to the beginning of the Carboniferous Period, about 359. 2 ± 2. 5 Mya.

There is a belt of younger Silurian rocks along the west coast between Niarbyl and St Patrick's Isle and a small area of Devonian sandstones around Peel.

It follows the Cambrian Period and is followed by the Silurian Period.

It is the longest of the Phanerozoic eras, and is subdivided into six geologic periods ( from oldest to youngest ): the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian.

The base of the Silurian is set at a major extinction event when 60 % of marine species were wiped out.

However, only one species is known from each of the Ordovician and Silurian periods, with a few more known from the Carboniferous ( Mazon Creek ).

It is not clear when the transition to a terrestrial existence was made, but it is considered plausible that it took place between the Ordovician and late Silurian — approximately — via the intertidal zone.

He is best known for pioneering faunal analysis of Silurian beds by means of index fossils, especially graptolites, and his proposal ( eventually adopted ) that the beds between the Cambrian beds of north Wales and the Silurian beds of South Wales should be assigned to a new geological period: the Ordovician.

Downpatrick is characterised by the rolling drumlins that are a feature of the Lecale area and a legacy of glaciation during the Pleistocene, the Down drumlins themselves are underlaid by Ordovician and Silurian shales and grits.

This class of arthropod is thought to be among the first animals to have colonised land during the Silurian geologic period.

The Silurian Tuscarora Formation, a hard sandstone, outcrops at the crests of these ridges and is stratigraphically lower and thus older than the rest of the bedrock in the township.

The overlying formations are either Silurian or Devonian in age, and include ( in ascending order ) the Clinton Group, the Bloomsburg Formation, the Mifflintown Formation, the Wills Creek Formation, the Tonoloway Formation, the Keyser Formation, the Old Port Formation, the Onondaga Formation, and the Hamilton Group, which is composed of the Marcellus Shale and the Mahantango Formation.

The Silurian Tuscarora Formation, a hard sandstone, outcrops at the crests of the ridges and is stratigraphically higher and thus younger than the rest of the bedrock in the valley.

The Silurian Tuscarora Formation, a hard sandstone, outcrops at the crests of the ridges and is stratigraphically higher and thus younger than the rest of the bedrock in the valley.

The type locality of the Silurian / Devonian Keyser Formation, a limestone, is located in a quarry and roadcut east of the town.

The dolomitic limestone is Silurian aged Niagaran Dolomite.

Reference is occasionally made to this period of Celtic history by the use of terms such as " Silurian ".

It is composed of the Lockport geological formation of Silurian age, and is similar to the Onondaga geological formation, which runs parallel to it and just to the south, through western New York and southern Ontario.

At lower elevations on the mountain, and stratigraphically below the Littleton Formation, the bedrock is from the Silurian period, consisting of Rangeley Formation schists, Perry Mountain Formation quartzites, the Francestown Formation granulite, and the Warner Formation granulite.

Most of the highest ground is formed by the masses of granite which have been intruded into the Ordovician and Silurian rocks ; the Criffel mass lies about Dalbeattie and Bengairn, another mass extends east and west between the Cairnsmore of Fleet and Loch Ken, another lies northwest and southeast between Loch Doon and Loch Dee and a small mass forms the Cairnsmore of Carsphairn.

The geologic period Silurian was first described by Roderick Murchison in rocks located in the original lands of the Silures, hence the name.

Gypsum has been dated to almost every geologic age since the Silurian Period 443. 7 ± 1. 5 Ma.

Deposition resumed in the Cambrian period and continued through the Paleozoic era, creating nine major formations which together are 4, 000 feet ( 1, 200 m ) thick ( the only geologic period in the Paleozoic not represented is the Silurian ).

* Ludlow epoch, part of the Silurian period in the geologic time scale

Category: Silurian geologic formations

In the 1840s fossils of brachiopods and trilobites from the Silurian period were discovered at Woolshed Creek near Duntroon.

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.

The Ordovician and Silurian periods were warm greenhouse periods, with the highest sea levels of the Paleozoic ( 200 m above today's ); the warm climate was interrupted only by a ~ 30 Ma cool period, the Early Palaeozoic Icehouse, culminating in the Hirnantian glaciation.

Xylem cells are specialised for conduction of water, and first appeared in plants during their transition to land in the Silurian period more than 425 million years ago ( see Cooksonia ).

The oldest known scorpions lived around 430 million years ago in the Silurian period, on the bottom of shallow tropical seas.

The oldest generally accepted shark scales are from about 420 million years ago, in the Silurian period.

Their numbers began to decline during the middle of the Silurian period, and they became extinct at the end of the Permian period,.

Rugose corals became dominant by the middle of the Silurian period, and became extinct early in the Triassic period.

Immediately above the topmost layer of the marine rock sequence forming Murchison's Silurian period was a thin layer of dark sand containing numerous remains of early fish, especially their scales, along with plant debris, spores and microscopic mites.

The later Silurian period saw the creation of limestone and shale, visible along the southern Marloes peninsula.

Neither Tethys oceans should be confused with the Rheic Ocean, which existed to the west of them in the Silurian period.

During the Silurian period, some magnesium substituted for some of the calcium in the carbonates, slowly forming harder ( dolomitic ) sedimentary strata in the same fashion.

The vertebrate jaw probably originally evolved in the Silurian period and appeared in the Placoderm fish which further diversified in the Devonian.

The vertebrate jaw probably originally evolved in the Silurian period and appeared in the Placoderm fish which further diversified in the Devonian.

The chief geological investigation of the last decade of his life was devoted to the Highlands of Scotland, where he wrongly believed he had succeeded in showing that the vast masses of crystalline schists, previously supposed to be part of what used to be termed the Primitive formations, were really not older than the Silurian period, for that underneath them lay beds of limestone and quartzite containing Lower Silurian ( Cambrian ) fossils.

The Silurian system was first identified by British geologist Sir Roderick Impey Murchison, who was examining fossil-bearing sedimentary rock strata in south Wales in the early 1830s.

Sir Roderick Impey Murchison, 1st Baronet KCB DCL FRS FRSE FLS PRGS PBA MRIA ( 22 February 1792 – 22 October 1871 ) was a Scottish geologist who first described and investigated the Silurian system.

The result was the establishment of the Silurian system under which were grouped, for the first time, a remarkable series of formations, each replete with distinctive organic remains other than and very different from those of the other rocks of England.

The establishment of the Silurian system was followed by that of the Devonian system, an investigation in which Murchison assisted, both in the south-west of England and in the Rhineland.

The greater portion of the county of Dumfries belongs to the Silurian tableland of the south of Scotland which contains representatives of all the divisions of that system from the Arenig to the Ludlow rocks.

* Sir Roderick Murchison KCB FRS, Scottish geologist who first described and investigated the Silurian system.

* Roderick Murchison-geologist, originator of the Silurian system

* Silurian, the geological period and system lasting roughly from 445 million years ago to 415 million years ago