MINERALOGY. 



437 



olcaoic. 



Geognosy, noes. They may be divided into the following orders. 

 1. Pseudo- volcanic. 2. Thermal, or those formed from 

 the water of hot springs. 3. Those formed from tor- 

 rents of hot water issuing from volcanoes. 4. Rocks 

 formed by torrents of mud flowing from subterranean 

 volcanic lakes. 5. Rocks formed by air or mud volca- 

 noes. 6. Volcanic. 



1. Pseudo-volcanic^ Pseudo- volcanic rocks are por- 

 tion* of previously existing strata, which have been 

 more or less altered by the action of heat emanating 

 from beds of coal in a state of combustion. The follow- 

 ing species are enumerated by geologists. 1. Burnt- 

 clay. 2. Porcdain-jatper. 3. Earlk-tlag. 4. Colum- 

 nar clou-iron ore. And 5. Poluhing-flate. 



1 . Burnt-clay. Its colour is usually red, and some- 

 time* grey, yellow, and brown, and occasionally spotted 

 or striped. Sometimes it encloses impressions of plants. 

 It is clay or slate-clay burnt, but not so much changed 

 as to form a porcelanous mass 



2. Porcelain-jasper. It is shite-clay, or common day, 

 changed into a kind of porcelain by the action of heat. 

 Lake the preceding species, it sometimes contains im- 

 pressions of plants, a fact which shews that it has not 

 been completely melted. 



3. Earth-flag. This is clay, or clay iron-stone, con- 

 verted into a kind of slag. It is black, brownish, or 

 reddish, and it has occasionally a tempered-steel tarnish. 

 It is amorphous or vesicular, and has sometimes me- 

 tallic lustre. 



4. Columnar clay iron-ore. This is clay iron-ore, 

 which is supposed to owe its columnar concretionary 

 form to the action of heat. 



5. Poluki*f-ilalf.l a grey or white coloured thin 

 slaty light mineral, which Werner conjectures to be the 

 ashes) of burnt coal, which have been carried by water 

 into low situations, and deposited in a slaty form. 



Situation. Pseudo-volcanoes usually occur in low 

 tkiMSrintii, and miminm also in hilly country, and al- 

 ways in rocks of the coal formation. 



Phenomena. They are discovered by the beat of the 

 surface of the earth in their \ icinity ; sometimes by 

 moke, and more rarely by flames issuing from rents 

 in the ground. Sulphureous and amm/mia^ vapours 

 frequently occur, and these, in their course upwards, 

 incrust the fissure* of rocks, and even the surt. 

 the ground, with sulphureous and ammoniacal matters. 



2. Thermal Rocki, or thou formed from Water of 

 Hot-Spnngi. 



San Filippo in Tuscany, have formed a hill of calcareous Geognoty. 

 tuffa, in many places as compact and hard as limestone. ""V"'' 

 The famous rock named iravertino by the Italians, and 

 which abounds in South-western Italy, is a product 

 partly of hot, partly of cold springs. The ancient tem- 

 nid the gorgeous palaces and churches of Rome, 

 and indeed the whole of the streets and squares of the 

 former Mistress of the World, are built of concretionary 

 masses which have been deposited by springs. 



There are many considerable hot-springs around 

 Guancavelica in South America, the waters of which 

 spread over the neighbouring country, and deposit up- 

 on it an ash-grey or whitish Mibstance, (calc-tuff, sinter 

 and travertine ?) which acquires a great degree of hard- 

 ness. The spring- water is so highly impregnated with 

 the earthy matter, that the inhabitants receive it in 

 square boxes or moulds, which it fills in a few days, 

 and the blocks, thus formed, are used for building. In- 

 deed the greater part of Guancavelica, like Rome, is 

 built of the concretionary rock formed from springs. 



3. Rock* formed from torrenit of Hot Water issuing 

 f from Volcanoes. 



There are on record authentic instances of torrents Root, 

 of hot water flowing from the crater or the sides of formed 

 volcanoes, when in a state of activity, which, when from tor- 

 collected in hollows, or spread over plains, deposites rcnu / hot 

 various earthy matters, which at length assume the " 



character of rocks. In the year 1 751, a torrent of salt 

 water burst from yEtna, and continued to flow for a 

 quarter of an hour, and was so considerable that the 

 inhabitants named it A'i/o d'Acyna. Dolomieu aiul 

 Hamilton observed traces of a frightful torrent of hot 

 water which had issued from the great crater of Alt na ; 

 and Spallanrani is of opinion, that part of the tutfas of 

 Italy have been formed by muddy eruptions. In those 

 volcanic mountains whose summits are above the snow 

 line, as is the case in Iceland and in South America, 

 great floods of hot water, charged with earthy matter, 

 burst from the mountains and devastate the surround- 

 ing country. Botiguer and Condaminc saw dreadful 

 ravages committed l>v tluse torrents; and the latter 

 - informs us, that after an explosion of Cotopaxi, 

 a village situated thirty leagues in a straight line, and 

 probably sixty leagues following the wavings of tin- 

 ground, was entirely carried away by one of those tor- 

 rents. These torrents from the ice-caped volcanoes, 

 appear to be entirely external, while those first men- 

 tioned seem to come from the interior of the volcanoes. 



Olafsen, Povelscn, Menge, and others, speak of ther- 



ofbasalt l>or- *' Rock* formed by Torrentt of Mud forcing from Suo. 



terranean Lakes. 



mal rocks having many of the characters _ _., ,,.- 

 phyry, wacke, \c. which occur in districts where for- 

 merly hot-springs existed, and even say they have ob- 

 served them actually forming around many of the mag- 

 nified hot-springs at present in a state of activity in 

 Iceland. These thermal trap-rocks, viz. thermal, basalt, 

 wacke, amygdaloid with calcareous-spar, porphyry, &c. 

 arc alleged to be brought from the interior of the earth, 

 by the water of hot-springs, partly in a state of solution, 

 partly in a state of mud, and are deposited over flat or 

 hilly tracts of country, when they gradually harden, 

 sometimes crystallize, and assume their various perma- 

 nent characters. The amygdaloidal traps appear to be 

 formed in those case* where carbonate of lime is pre- 

 sent ; the obsidian where dissolved silica prevail* ; and 

 in the same general way we might account for the for- 

 mation of the other rocks. 



The hot-springs of Carlsbad in Bohemia are of 

 this description, and the well known hot-springs of 



In the interior of volcanic mountains, there some- 

 times occur caverns or hollows partly filled with formed by 

 water, thus forming subterranean lakes. The earth- to"""' f 

 quakes that often agitate these mountains, are occa- " d * ow " 

 tionaliy so violent, as to produce great rents, which lubterTs*- 

 give free passage to the water of the Likes, hich n ein Ukw. 

 bursts forth with tremendous violence, deluges the 

 neighbouring country, and coven it to a greater or less 

 extent, and with a more or less deep crust of muddy 

 matter. In the earthquake of the year 174(1, which 

 overturned Lima, four volcanoes opened at Lucanas, 

 and in the mountains of Conception, and occasioned 

 frightful deluges. The volcanoes of the kingdom of 

 Quito sometimes exhibit phenomena of the same kind, 

 but accompanied with circumstances so extraordinary, 

 that we shall now state them. The enormous volcanic 

 of Cotopui, Pichincha, Tungoursgua, &c. in 



