438 



MINERALOGY. 



Geognosy. South America, never throw out lava, but frequently 

 ^' i-~* ashes, scoriae, and pumice, and sometimes vomit forth 

 immense quantities of water and mud. These erup- 

 tions take place more frequently from the sides than 

 from the craters of the volcanoes, and the muddy wa- 

 ters appear to be derived from lakes situated in the in- 

 terior of the mountains, which burst forth with incre- 

 dible fury, when any accidental cause, such as an earth- 

 quake, splits, and thus opens the side of the mountain. 

 In the year 1698, the mountain of Carguarazo, near to 

 Chimboraco, fell down and covered eight square leagues 

 of country with mud. In the earthquake of the 4th 

 February 1791, 40,000 persons were destroyed by 

 eruptions of water and mud, (moya). Muddy waters, 

 resembling those which flow from volcanic mountains, 

 are vomited forth in great quantity, from districts where 

 no volcanic rocks occur, when these are agitated by 

 earthquakes or other causes. In Peru and Quito, the 

 devastations occasioned by volcanoes are not caused by 

 streams of lava, but by water and enormout streams of 

 mud, which, when hardened, is found to contain cry- 

 stals of felspar, and to resemble porphyry. 



5. Rocks formed by Air or Mud Volcanoes. 



Rocks In some countries, jets or great bubbles of water, 



formed by highly impregnated with mineral matter, are thrown 

 air or mud ou t o f the earth by means of gas. The earthy matter 

 Tolcanoes. j s deposited in the state of mud, principally around the 

 mouths of the cones from whence it is expelled ; and 

 as these cones somewhat resemble volcanoes in form, 

 they have been named Air-volcanoes. One of the most 

 remarkable of these air-volcanoes hitherto described, 

 is that of Macalouba in Sicily, of which an account 

 has been published by Dolomieu. It consists of a 

 hillock of hardened mud, about one hundred and fifty 

 feet in height. Its superior part forms a plain more 

 than half a mile in circumference, and rising from it 

 are numerous small cones not more than three feet in 

 height, each of which has a crater or hollow filled more . 

 or less deeply with a liquid mud, which is in a state of 

 perpetual agitation, owing to the constant passage of 

 great bubbles of air through it. Portions of the mud 

 are constantly thrown out, and thus add to the bulk 

 and height of the cones. 



There are many small mud-volcanoes in the neigh- 

 bourhood of Modena, whose height is not more than a 

 few feet. They are named Salses, on account of the 

 saltness of the water they throw out ; and which, in- 

 deed, is also the case with the water of. Macalouba, 

 and of that of most other muddy eruptions observed in 

 different countries. These volcanoes, during their pa- 

 roxysms, which are attended with slight agitations of 

 the earth, throw out much mud, which extends to the 

 distance of three thousand feel. The gas which occa- 

 sions the eruptions, is sulphuretted hydrogen mixed 

 with petroleum, and a little carbonic acid. Similar 

 air-volcanoes are described by Pallas as occurring in 

 the Crimea, particularly in the Island of Taman. In 

 the year 1794, one of these burst with a noise like that 

 of thunder, and flame and smoke rushed from it to the 

 height of more than three hundred feet. Great masses 

 of dried mud were projected from it to great distances, 

 and it vomited forth currents of a bituminous mud or 

 slime, to the amount of one hundred thousand cubic 

 fathoms. Humboldt describes air-volcanoes which he 

 saw in the middle of an elevated plain, in the province 

 of Carthagena in South America. There were twenty 

 small cones, having an elevation of from twenty-one to 

 twentyseven feet in height, and formed of bluish colour- 



ed clay. Their summits were hollow or crater-like, Geognosy 

 filled with water, from the surface of which air arose, **-> ' 

 and burst with an explosion, and often threw out mud. 

 In the Island of Trinidad, and also in Java, there are 

 considerable air or mud volcanoes. 



6. Volcanic Rocks. 



These are the mineral substances formed or thrown Volcanic 

 out by true volcanoes when in a state of activity. The rocks, 

 following are the most characteristic of these substances. 

 1. Lava; 2. Tuffaj 3. Volcanic ashes; 4. Volcanic 

 glass. 



1. Lava. Many different kinds of lava are enume- 

 rated by geologists, and of which a particular account 

 will be given in the article VOLCANO. For our pre- 

 sent view it will be sufficient to arrange them under 

 the following heads, viz. : Compact, vesicular, slaggy, 

 and spumous. 



a. Compact Lava. Colour grey ; massive, and some- 

 times in columnar distinct concretions. Lustre glim- j om P ac 

 mering. Fracture uneven or splintery. Opaque. 

 Semi-hard, approaching to hard. Brittle and easily 

 frangible. Feels dry and rough. Sp. gr. = 2.80. 



It always occurs in the form of streams, (coule,) and 

 generally forms the middle, part of the stream. 



b. Vesicular Lava. Colours grey, black, and brown. Vesic 

 Structure vesicular. Generally occurs in the upper, Java 

 and also on the under sides of streams of lava, when 

 they have run over moist ground. 



c. Slagsy Lava. Colour black or brown, and has a $| ag gj 

 completery slaggy aspect. It occupies the Uppermost i V a. 

 part of lava streams. 



d. Spumaceous Lava. Colour red or brown, and so Spuma- 

 very vesicular that it sometimes will float in water, ceou. lava. 

 Like slaggy lava, it always occurs on the surface of 

 streams. 



2. Tnffa. Is a conglomerated volcanic rock, with T uffa< 

 an earthy basis, including masses of different kinds of 

 lava, volcanic glass, &c. 



3. Volcanic Ashes, -These are the loose powdery v o , Mni 

 earthy matters thrown out when volcanoes are in a state as h es . 

 of activity. 



4. Volcanic Glass. Certain varieties (if not all) of V8 , canic 

 obsidian, described at page 54-9, are to be considered as ^ ass 

 volcanic glasses. These are lavas which have been in 



a state of perfect fusion. 



* VEINS. 



Veins are tabular-shaped masses, that almost always . 

 traverse the direction of the strata, and are composed 

 of materials that either differ more or less from those of 

 the rocks which they intersect, or are of the same na- 

 ture. 



1. External Relations, of Veins. 



Breadth and Extent of Veins. Veins vary very 

 much in their magnitude, yet the length and depth al- 

 ways bear a certain proportion to each other, and the 

 breadth to the length and depth. The length and depth 

 are frequently nearly alike, yet the length, onja general 

 view, may be considered as generally somewhat more 

 considerable than the depth. 



1. Breadth or width of Veins. In. most metalliferous 

 mountains, we find that metalliferous veins extend a wid(h Qf 

 few hundred fathoms, and then their width does not Teing 

 exceed two feet. 



Veins whose width exceeds a few fathoms are to be 

 considered as uncommon, and those whose width is 

 still greater are to be viewed as exceptions to their ge- 

 neral appearance. Humboldt and Friesleben observed 



