834

VOLCANIC PRODUCTIONS.

Like puzzolaua, this substance is useful in the preparation ofmortar. It was employed by Mr. Smeaton for this purpose in theconstruction of the Eddystone Light-house.

12. Volcanic Tufa. This name is applied to several differentsubstances, some of which have never suffered the action of fire, al-though they have proceeded from volcanoes.

1. This tufa is sometimes an aggregate of sand, volcanic ashes,and fragments of scoria and lava, united by an argillaceous or muddycement.

2. Sometimes it is composed of volcanic ashes and sand, trans-ported and deposited by rain-water. Such tufas are constantly form-ing in volcanic countries.

3. The earthy deposite, which proceeds from muddy eruptions,has also received the name of volcanic tufa.

Tufa presents various shades of gray, brown, red, yellow, &c. oris spotted. It has a variable, but moderate, degree of hardness ; andits fracture is earthy and dull. It has sometimes a uniform texture,and sometimes it embraces fragments of limestone, basalt, hornblende,schorl, feldspar, &c. &c.—The tufa of the Solfaterra, near Naples,sometimes contains impressions, or even the leaves, of a species ofsea-weed. ( Spallanzani .)

It is easy to suppose, that eruptions of slimy earth and muddywater may occasionally proceed from reservoirs in the sides or bodyof volcanic mountains. But it appears, that many of the volcanoesof South America often discharge vast quantities of water, and of anearthy, slimy substance, called Moya by the natives, and Koth bythe Spaniards. In addition to this Moya, these volcanoes eject ash-es, pumice, and slags, hut seldom any lava.

Moya has a blackish brown color, an earthy texture, and but littlecoherence. It contains fragments of feldspar, and frequently alsogreat numbers of fishes (Fimelodi Cyclopum). Some varieties arecombustible, and burn without flame.—A specimen yielded Klaproth silex 46.5, aluminel 1.5, water, containing ammonia and empyreumat-ic oil, 11.0, oxide of iron 6.5, lime 6.25, coal 5.25, soda 2.5, hidro-gen gas 14.5 cubic inches, carbonic acid 2.25 cub. inches.

A similar current of slimy, argillaceous earth appears to haveonce flowed from a volcano in the island of Lipari. (Spallanza-ni.J

Some naturalists suppose this water to be derived from the sea orfrom lakes by subterraneous passages ; and that the eruptions of vol-canoes are effected chiefly by the expansion of aqueous vapor. Thenecessary heat may be furnished by the combustion of immense bedsof coal.