436 



MINERALOGY. 



Geognosy, continue to drift that which is at the summit further 

 S -"~Y" < *'*' towards the land. 



b. Clays. 



Clayt. Many extensive plains are covered with clays and 



loams of various descriptions; these also occur fill- 

 ing up hollows in plains, and forming extensive tracts 

 on the sides of lakes, and at the mouths of rivers 

 where they enter lakes, or meet with the waters of the 

 ocean. These clays are formed by the disintegration of 

 felspar, micaceous, and slate rocks. 



1 . Substances that occur in the sand and clay. 



Substances These are, 1. Calcareous tuff, 2. Bog-iron ore, S. 



that occur Ores, metals, and gems in grains, 4. Common salt, 5. 



nd'ci 8 *"' 1 Subterranean and submarine forests, 6. Peat. 



Calcareous ' Calcareous tujf. There are two kinds of this 



tuff. rock, one of old formation, and another which is daily 



forming. The first appears to have been deposited 

 from the waters of lakes that formerly existed in lime- 

 stone districts, but which have long since disappeared. 

 Thuringia affords striking examples of this formation 

 of calc-tuff. The tuff there jests upon gravel, or on 

 the rocks of which the country is composed : it forms 

 beds sometimes upwards of fifty feet thick, and which 

 are composed of strata of compact and porous tuffa, 

 which frequently alternate with each other, and be- 

 tween them are sometimes thin beds of a brown bi- 

 tuminous earth. When the lakes contain no plants, 

 then the tuff deposited in it is always compact, but if, 

 on the contrary, they abound in reeds, rushes, confer- 

 vas, &c. the tuff, owing to this intermixture, is po- 

 rous, and loose in its texture. These tuffs in Ger- 

 many contain osseous remains of elephants, rhino- 

 ceroses, megatheria, deers, &c. and sometimes also re- 

 mains' of fresh water shells, analogous to the species 

 at present met with, and also impressions of indigenous 

 plants ; and very lately fossil human skulls are said to 

 have been met with in this formation. 



The waters which flow along the surface of the 

 globe, and which are charged with calcareous earth, de- 

 posit it on the districts they traverse, and thus form 

 tuffas, which are either porous or compact. An ex- 

 ample of this formation occurs at Starlyburn near Burnt- 

 island in Fifeshire. 



2. Bog-iron ore. 



Bog-iron Water, in its passage over and through rocks con- 



ore, taining iron, abstracts a portion of it ; it also carries 



away the iron contained in the beds of decaying vege- 

 table matter it passes through, and in both cases a por- 

 tion of the water combines with the iron, so that when 

 the iron comes to be deposited on the bottom of lakes, or 

 spread over the flat country, it appears in the form of 

 hydrate of iron, and thus gives rise to the beds of bog- 

 iron ore, met with in alluvial districts. In many dis- 

 tricts in Scandinavia, where the primitive strata are very 

 richly impregnated with iron, bog-iron ore is found in 

 great abundance in the bottom of lakes, and so rich as 

 to yield 60 per cent, of hydrate of iron. The iron is 

 dredged out in some instances every ten, in others eve- 

 ry twenty and thirty years, thus shewing the rapid re- 

 newal of the deposite. Some lakes or morasses, the 

 bottoms of which are covered with these ferruginous 

 deposites, dry up, and become covered with a bed of 

 soil and grass, and thus form meadow-ore ; Wer- 

 ner gives the following account of the formation of 

 this ore of iron, as observed by him in Lusatia. He 

 distinguishes three principal varieties, which he re- 



fers to three periods of formation ; that which is ob- Geognosy. 



tained from swamps, and which is still soft at the time ""-Y""' 



of its being removed he names sn>amj)-ore ; that which 



is hard and covered with marsh plants is his marsh-ore; 



and lastly, that which is dug from districts entirely dry 



and covered with a thin layer of soil, is named mea- 



dotv-ore. 



3. Ores, metals, and gems in grains. 



Some kinds of alluvia, particularly those in moun- Ores, me- 

 tainous districts, contain masses and grains of ores and tals > a . nd 

 native metals, which are derived from metalliferous Ber ? 8 m 

 veins, metalliferous beds, or from rocks through which gral 

 the metalliferous minerals have been disseminated. If 

 the minerals yield easily to decomposition and tritura- 

 tion, they are reduced to very minute particles, and 

 become intimately mixed with various clays and loams ; 

 but if their hardness and tenacity is such that they re- 

 sist obstinately, then they appear in rolled pieces, and 

 in grains of various sizes and forms. The ores and me- 

 tals most frequently met with in this situation, are iron- 

 pyrites, tin-ore, native gold, and platina. The three 

 latter are those which, on account of their value, are 

 extracted from the sands and clays by washing. 



The washing for gold is practised in many countries, 

 and in some, as in Brazil, to a great extent. Indeed, 

 nearly three-fourths of the gold of commerce are ob- 

 tained from sand by washing. 



Platina is obtained by washing the alluvial soil of 

 the province of Choco in Peru ; and there it is accom- 

 panied with fragments of basalt, zircon, titanium, 

 iron sand, and grains of gold. The metals known un- 

 der the names palladium, rhodium, osmium, and iri- 

 dium, are also obtained from the sands of Choco. 



Tin-ore is found in considerable abundance in allu- 

 vial soil in Cornwall, Saxony, and in the island of Ban- 

 ca. 



Many of the gems are obtained by washing alluvial 

 sands and clays; the pyrope or garnet of Bohemia, 

 zircons, rubies, topazes, and diamonds, are collected in 

 alluvial districts. 



4. Common Salt. 



Rock-salt occurs in great abundance in the deserts Common 

 of Africa and Arabia, the plains of Persia, and the sa i t . 

 steppes of Siberia. 



5. Subterranean and Submarine Forests. 



Sometimes whole forests are found covered with al. Subterran- 

 luvial deposites, and these are either under alluvium on ea 

 the dry land, or extend under the waves of the ocean. '^^ 

 The first are denominated subterranean, and the latter 

 submarine forests. There is an extensive subterranean 

 forest in Lincolnshire, and another in Lancashire ; and 

 on the coast of Lincolnshire there is an immense sub- 

 marine forest, which has been particularly described by 

 Mr. Correa. The sinking and sliding of the alluvial 

 strata, and the breaking down of natural sea-barriers 

 on front of flats, marshes, and lakes, easily explain all 

 the phenomena exhibited by submarine forests. 



6. Peat. 



There are six different kinds of peat, viz. moun- Peit. 

 tain, marsh, lake, forest, marine, and transported, all of 

 which we shall describe in our natural and economical 

 history of this substance under the head PEAT. 



CLASS V. VOLCANIC ROCKS. 



We include under this class all those rocks which are Veltanic 

 alleged to have been formed by the agency of volea- rockt. 



