ON GEOLOGY. 



69 



of which is generally found in a particular situation : sandstone of different 

 kinds, and differently arranged, three sets ; limestone, 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 forma- 

 tions : the relative position of the rest is more variable. The fioetz or hori- 

 zontal 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 ; in- 

 dicating 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 denomi- 

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

 earth's solid crust. They have been evidently produced by the gradual ac- 

 tion of rain, river-water, air, and the elastic gases, upon the other classes, and 

 may, comparatively, be considered as very recent formations, or rather as de- 

 posites, 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 mountains, and those deposited upon flat 

 land. 



The first kind consists of sand, gravel, and similar materials, which consti- 

 tuted 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 often washed in order to se- 

 parate 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 fiat land, 

 consists of loam, clay, sand, marl, calcsinter, and calctuff, or stalactitic 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 defending them from 

 the action of the air. The clay and sand sometimes contain petrified wood ; 

 and in many part« 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 (in which is often traced mineral amber), wood- 

 coal, bituminous wood, and bog iron ore. 



The LAST, or uppermost, of the five classes of rocks of the Wernerian sys« 

 tem, is denominated volcanic 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 

 minerals which are thus altered are porcelain, jasper, earth, slag, burnt-clay, 

 columnar clay, ironstone, and, perhaps, polishing slate. 



The real volcanic minerals are those which have been thrown out of the 

 crater of a volcano, and consist of three kinds : first, those which, having 

 been discharged frequently, have formed the crater itself of the mountain; 

 secondly, those which have rolled down in a stream, and are known by the 

 name of lavas : and, thirdly, the residual matter contained in the water which 

 is often ejected, composed of ashes and other light substances, and which, 

 when rendered solid by evaporation, is denominated volcanic tuff or tufa. 



I have observed that these different classes of mineral formations are often 

 traversed in various directions by other mineral substances which are called 



♦ See series II. lect. ii. On zoological systems, and the distinctive characters of animals. 



