ON GEOLOGY. 



diateiy over the transition-class, and consist of the twelve following distinct 

 sets of rock, each of which is generally found in a particular situation : 

 sandstone of different kinds, and differently arranged, three sets ; lime- 

 stone, three sets ; gypsum, two sets ; calamine ; chalk ; coal ; trap. The 

 trap usually covers the whole of this class, as the newer porphyry and sienite 

 cover the primitive formations : the relative position of the rest is more 

 variable. The floetz or horizontal class is characterized by its containing 

 an abundance of petrifactions in every one of its sets, and these of known 

 animal and vegetable kinds ; though still, of those that occupy the lower 

 parts of the scale, as shells, fishes, the fishes much mutilated, a few tortoises, 

 ferns, pines, and reeds ; indicating that they were formed at a period in 

 which organized beings of this character abounded, but in which those of 

 other characters did not exist, or but rarely. 



The FOURTH CLASS of formations, under the Wernerian system, is de- 

 nominated ALLUVIAL, and constitutes the great mass of the actual surface 

 of the earth's solid crust. They have been evidently produced by the 

 gradual action of rain, river-water, air, and the elastic gases, upon the 

 other classes, and may, comparatively, be considered as very recent for- 

 mations, or rather as deposites, whose formations are still proceeding. 

 They may be divided into two kinds ; those deposited in the valleys of 

 mountainous districts, or those elevated plains which often occur in moun- 

 tains ; and those deposited upon flat land. 



The first kind consists of sand, gravel, and similalr materials, which 

 constituted part of the neighbouring mountains in their original state, and 

 which remain, notwithstanding that these less durable parts have been 

 thus washed or blown away. They sometimes contain ores, which also 

 existed in the neighbouring mountains, and have been carried down by the 

 agency of rain, air, or the elastic gases. The ores principally discovered 

 in such situations are those of gold and tin ; and these soils are oflen 

 washed in order to separate them. Beds of loam are also occasionally 

 met with on the plains of mountains, formed of the decomposed elements 

 of animal and vegetable bodies that once occupied their sides. 



The second kind of alluvial deposites, or that which occupies the flat 

 land, consists of loam, clay, sand, marl, calcsinter, and calctufF, or sta- 

 lactitic tufa, the basis of our common petrifactions ; and which is found 

 very largely in Sweden, Germany, and Italy, clothing with a calcareous 

 coat the smaller branches of trees, leaves., prickles, moss, and other minute 

 plants ; eggs, birds, and birds-nests ; preserving them from decay, by de- 

 fending them from the action of the air. The clay and sand sometimes 

 contain petrified wood; and in many parts are found the skeletons of 

 quadrupeds, even of the largest magnitudes, as we shall have occasion to 

 observe hereafter.* Here, also, occur earths and brown coal (m which 

 is oflen traced mineral amber), wood-coal, bituminous wood, and bog iron 

 ore. 



The LAST, or uppermost, of the five classes of rocks of the Wer- 

 nerian system, is denominated volcakic formations ; and consists of two 

 distinct sets, false and true. 



The false comprise mineral substances which have experienced a change 

 from the combustion of beds of coal situated in the neighbourhood : the 

 chief materiajs which are thus altered are porcelain, jasper earth, slag, 

 burnt-clay, columnar clay iron-stone, and, perhaps, polishing slate. 



• * See Sfiiies II, Lect. II. On zoological systemsj and the distinctive cliaracters of animals 



