(2) More common are the auriferous quartz-reefs - veins or masses of quartz containing gold in flakes visible to the naked eye, or so finely divided as to be invisible.
The Rand reefs, first mined in 1886, cover a large area.
They are apparently volcanic. Coral reefs lie off the coasts and render them difficult of access.
The formation is noted for its regularity as regards both the thickness and the gold-tenor of the ore-bearing reefs, in which respect it is unparalleled in the geology of the auriferous formations.
It is surrounded by coral reefs, and is richly wooded.
Round most of the islands there is a luxuriant coral growth; but, as the reefs lie at no great distance, and follow the line of the coast, the inter-island channels are comparatively safe.
No auriferous reefs have been found.
This was a conservative estimate, and was made before the full extent of the reefs was known; in 1904 Lionel Phillips stated that the main reef series had been proved for 61 m., and he estimated the gold remaining to be mined to be worth £2,500,000,000.
The discovery of alluvial gold in 1851 brought many immigrants to the district; but the opening up of the quartz reefs in 1872 was the principal factor in the importance of Bendigo.
In the Mwanza district are conglomerate reefs of great extent.
Coral reefs protect the coasts in many parts; they are frequently interrupted, but the passages through them are often difficult of navigation.
To the north the reefs continue, marking the former extension of the land, for about 160 m., ending with the Huon Islands.
Its coasts are encircled with coral reefs, extending in some places 3 m.
The quartz reefs which crop out in the granite ranges of the Tehama contain traces of gold.
The coast-line is fringed with small islets and shoals and reefs, which make navigation dangerous.
The principal formation is coralline limestone; the eastern coast is defended by coral reefs, and the neighbouring sea (extending as far as New Guinea, and thus demonstrating a physical connexion with that land) is shallow, and abounds in coral in full growth.
Here the reefs are generally less perfect than elsewhere, seldom forming complete central lagoons, and as they were formerly exposed to the constant attacks of the Mopla pirates from India, the people are hardier and more vigorous than their less warlike southern neighbours.
Minikoi atoll, with the numerous wrecks on its reefs, its lighthouse, and its position on the track of all eastward-bound vessels, is a familiar sight to seafarers in these waters.
The Kevori grits, and the raised coral reefs are upper Cainozoic, and perhaps Pleistocene; but the reefs occur inland up to a height of 2000 ft.
The group, consisting of small islands and reefs (which mark the extreme northern range of the coral-building polyps), is of oval form, measuring 22 m.
Even the reefs are not wholly formed of coral.
As in other countries, however, the working of quartz reefs gradually compensated for this.
The lodes occur in Silurian metamorphic micaceous schists, intruded by granite, porphyry and diorite, and traversed by numerous quartz reefs, some of which are gold-bearing.
The Sciacca reefs were again closed for three winters by a decree of 1904.
The coasts are fairly indented, and, protected by these reefs, which often support a chain of green islets, afford many good harbours and safe anchorages.
Barrier reefs are rare; fringing reefs are numerous, except on the east side, which is nearly free, and there are many small isolated reefs and volcanic banks among the islands.
'hlatherv i W A WoodVillef w Scale, 1:2,200,000 English Miles 20 30 40 Longitude Nest gi of Greenwich z fishery on the reefs in the Sound, much developed since 1880.
Desis lives invariably between tide-marks upon the rocks and coral reefs, and may be found at low tide either crawling about upon them or swimming in tidal pools and feeding upon small fish or crustaceans.
Besides these there are a number of small indentations, sheltered anchorages formed by islands and reefs like that of Puerto Cabello, and estuaries and also open roadsteads, like those of La Guaira and Carupano, which serve important ports.
It appears to consist in the main of a continuation of an axis of old schists and slates, with granite intrusions, and flanked by coastal plains with Cretaceous or Jurassic, and Miocene beds, with Pleistocene sands and reefs and volcanic rocks.
Many ships have been wrecked on the jagged reefs which fringe their base.
If the reefs impede navigation they form some good harbours.
Along much of the western coast and along nearly the whole of the eastern coast extends a line of sand reefs and narrow islands, enclosing shallow and narrow bodies of water, such as Indian river and Lake Worth - called rivers, lakes, lagoons, bays and harbours.
On account of its sand reefs, the east coast has not so many harbours as the west coast.
It issues from the south-west corner of Lake Ladoga in two channels, which are obstructed by sandstone reefs, so that the better of the two has a depth of only 7 to 16 ft.
The coast line, including larger bays, but excluding reefs, islets, keys and all minute sinuosities, is about 2500 m.
Side of the island there are long chains of islets and reefs and coral keys (of which it is estimated there are 1300), which limit access to probably half of the coast, and on the N.
It is the centre of an important gold-field, the reefs of which improve at the lower depths, the deepest shaft on the field being 2558 ft.
The coral reefs are of especial interest from their bearing on the general question of the formation of coral reefs.
Agassiz, "A Reconnaissance of the Bahamas and of the Elevated Reefs of Cuba in the steam yacht ` Wild Duck,' January to April 1893," Bull.
Coral reefs remove calcium from solution in the sea on a vast scale.
It is not at all certain that the masses on which coral reefs are built consist entirely of the remains of the skeletons of reef-forming organisms and it is probable that chemically precipitated carbonate of lime predominates.
Such material, it is suspected, may form the massive bases on which barrier or fringing or atoll reefs are built up.
Interesting speculations as to the periods of origin of great coral reefs have been made by Wayland Vaughan, Andrews and Daly and Humphreys.
In Pleistocene times, then, when there were prolonged glacial ages, the sea-level was lowered and at the same time there was a reduction in sea temperature, so that the rate of reproduction of the coral polypes, and so the growth of reefs, was diminished.
Corals would now grow luxuriantly in these shallow coastal waters of increasing temperature, forming reefs and extensive coral flats.
In so far as it depends on solution of calcareous rock the Semper-Murray theory of coral reefs is unsatisfactory.
Recent papers on coral reefs are published in the Annual Report of the Smithsonian Institution, 1917; and F.
The central plateau is a plain whose surface presents "rounded, flat-topped hills and low ridges and reefs of limestone," with narrow intervening valleys.
The Aughrabies or Hundred Falls, as they are called, are divided by ledges, reefs and islets, the last named often assuming fantastic shapes.
The remarkable " stone reefs " of the north-east coast are ancient beaches hardened by the infiltration of carbonate of lime.
They are quite distinct in their formation from the coral reefs of the same coast.
The others are either difficult of access, or are rendered practically useless by dangerous reefs, sand bars and shoals.
The principal gold-bearing reefs are found along the Witwatersrand (" The Rand ").
Reefs are the gold-bearing rocks in the Klerksdorp, Potchefstroom and Venterskroon districts.
Other auriferous reefs are found all along the eastern escarpment of the Drakensberg and are worked in the De Kaap (Barberton) district, on the Swaziland frontier, in the Lydenburg district, in the Murchison Range and in other places in the Zoutpansberg.
The reefs are narrower than those of the Rand, and the, ore is usually very hard.
About this time gold reefs were discovered in the Zoutpansberg district near Marabastad, and a few gold seekers from Europe and Cape Colony began to prospect the northern portions of the Transvaal.
Under favourable conditions mining may be conducted under the protection of a few yards of solid rock only, as in the submarine work for the removal of reefs in the harbours of San Francisco and New York.
There are two harbours,difficult of access owing to the number of reefs and sunken rocks.
The islands on the southern margin of the Louisiade Archipelago are raised coral reefs, but the majority are mountainous, rarely, however, exceeding 3000 ft.; all of them are richly forested, but of little agricultural value.
Gold is obtained chiefly from quartz reefs, but there are still some important alluvial deposits being worked.
The material is largely calcareous, and has probably been derived from the disintegration of the reefs, and from the shells of animals living in the shallows.