426 



MINERALOGY. 



Graphite. 



Arrange- 

 ment of 

 the beds in 

 the coal 

 formation. 



Identity of 

 character 

 of the for- 

 mation in 

 different 

 countries. 



Stratifica- 

 tion. 



Situation. 



Dikes. 



and England, and affords nearly all -the iron of com. 

 merce produced in Great Britain. 



7. Clay. The different kinds of fire-clay, so well 

 know/i in the arts, occur in beds in the coal formation. 



8. Trap and Porphyry. Different species of these 

 rocks are met with in the coal-fields of Scotland, and 

 of other countries. We have observed the following, 

 viz. Greenstone, Amygdaloid, Basalt, Trap-luff, and 

 porphyry, in beds and in veins varying in magnitude 

 and extent. 



-, 9- Graphite, or Black Lead Beds of graphite occur 

 but rarely in this formation. There are examples of 

 this arrangement in Ayrshire. 



Arrangement of the beds in the Coal Formation. 

 Although no very regular arrangement of the beds 

 occurs in this formation, nevertheless it has been re- 

 marked that in some districts beds of coal are general- 

 ly contained in the slate, and that as we recede from 

 the coal the slate becomes coarser and coarser, and at 

 length passes into sandstone ; in others, the sandstone 

 most generally forms the floor, while the roof is of slate : 

 and in others the coal is covered with trap or lime- 

 stone, or rests upon these rocks. 



In many coal-fields the beds of coal and their accom- 

 panying rocks are frequently repeated in precisely the 

 same order, and in nearly the same thickness. 



The number of beds of coal superimposed on each 

 other in the same field is very considerable. At New- 

 jcastle, twenty-five beds have been penetrated in sink- 

 ing pits. The hill of Dutweiler in Saarbruck, con- 

 tains thirty-two beds ; at Liege, there are sixty beds, 

 and at Anzin more than fifty. 



Identity of Character of the Formation in different 

 countries In Scotland, the predominating and cha- 

 racteristic members of the formation are sandstone, bi- 

 tuminous shale, slate clay, clay ironstone, and coal. 

 In England we find in all the coal-fields precisely the 

 same sandstones, slates, ironstones, and coals, as in 

 Scotland ; and the same is the case in all the coal 

 mines of France, Germany and America. Everywhere 

 we meet with the same rocks, the same vegetable im- 

 pressions, and the same general arrangements of the 

 different rocks of the formation. 



Stratification All the rocks of this formation are 

 stratified, some more, and others less, distinctly. The 

 most perfectly stratified are the sandstone and slate j 

 and those having this structure in the most imperfect 

 degree, are the trap and porphyry rocks. The strata 

 often follow every inequality of the fundamental rock 

 on which they rest j and as the surface of the funda- 

 mental rock is frequently remarkably uneven, the su- 

 perimposed strata acquire a very irregular and contort- 

 ed aspect. In some cases the strata do not appear 

 to follow the inequalities of the fundamental rock, but 

 have directions that appear independent both of the 

 surface of the rock, and of the cavity or hollow, in 

 which they are contained. 



Situation. The rocks of this formation are general- 

 ly situated at the foot of mountains, or in bason or 

 trough-shaped hollows, which vary from a few hun- 

 dred yards to many miles in extent. It seldom rises 

 high above the level of the sea, and the countries it 

 forms have generally a waved and soft outline. In this 

 island it rests either on the mountain limestone or red 

 sandstone, and is covered by magnesian limestone and 

 other newer formations. 



Dikes. The strata of this coal formation are fre- 

 quently traversed by veins, which are composed of earthy 

 minerals, and are named Dikes. These dikes are 



I 



sometimes only a few inches wide, and not many fa- Geognosy., 

 thorns in extent ; in other cases they are upwards of V "V"' 

 one hundred feet wide, and extend for some miles. 

 Their direction varies, as also their angle of inclination, 

 which latter ranges from the nearly horizontal to the 

 vertical position. The strata in the walls of the dikes, 

 in some cases, correspond on opposite sides, while in 

 others, the corresponding strata on the hanging side 

 are depressed some feet, or even fathoms, so that min- 

 ers, in working a bed of coal for example, when stop- 

 ped in their progress by a dike, do not find it directly 

 opposite on cutting through it, but some feet or fathoms 

 out of the line of bearing of the bed. When the 

 strata present such an appearance, they are said to be 

 shifted. The materials of these dikes, or veins, varies, 

 as appears frorfi the following enumeration of the rocks 

 of which they are sometimes formed : Greenstone, amyg- 

 daloid, trap-tuff, porphyry, sandstone, and fragments 

 of the various surrounding strata. The strata, where in 

 contact with dykes, appear sometimes of a different na- 

 ture from the other parts of their mass ; thus beds of 

 coal, where in contact with the dike, appear as if char- 

 red, slate-clay hardened, and sand and lime indurated ; 

 yet in the midst of these apparently changed portions 

 there occur, unaltered minerals, such as calcareous 

 spar, iron pyrites, and clay. 



Metalliferous Minerals. The ores most frequently Metai 

 met with in this formation, are clay iron ore, and ga- ferous mi 

 lena, or lead glance. The iron ore occurs every where nerals. 

 in the coal-fields of this island ; but the lead-glance is 

 found in quantity only in the coal-districts in the north 

 of England, and in Wales. Copper, silver, and even 

 gold, are enumerated among the metalliferous produc- 

 tions of this formation ; and it would appear that cin- 

 nabar or sulphuret of mercury, is sometimes also con- 

 tained in it. 



Petrifactions. Mineralized organic remains are not Petrifac- 

 unfrequent in the coal-fields of different countries ; and tions. 

 it is worthy of remark, that hitherto the same tribes 

 and species have been met with in the coal formations 

 of Great Britain, Ireland, Germany, and France. Both 

 vegetable and animal remains occur, but of these the 

 former are by far the most varied and abundant. The 

 impressions of plants are frequent, both in the slate- 

 clay arid bituminous-shale ; and we cannot recollect a 

 coal mine where these rocks have not been foqnd to 

 contain organic vestiges of this description. Many be- 

 long to the monocotyledonous class of plants, and are 

 principally aquatic, such are the large reeds and bam' 

 boos ; besides these, we meet with numerous remains 

 of ferns, and species of the genera lycojiodiwn, equisi- 

 turn, euphorbia, casuarina, Sfc. Impressions of the 

 branches and fruit of palms, or of vegetables resembling 

 this tribe, are also occasionally met with. None of 

 these plants are identical with any of the present known 

 living species; and many of them have a tropical aspect. 

 These vegetable remains seldom occur in \he coal, 

 but are abundantly distributed in the slate, particularly 

 where it is near the surface of the bed of coal ; and 

 the most frequent remains are those of leaves or of 

 flattened trunks, sometimes changed into coal, enclosed 

 in the layers of the slate ; but in other instances the 

 reeds, and other vegetables of large diameter, are up- 

 right, and are filled with clay, or with the same sub- 

 stances as that in which they are contained. 



The animal remains found in the coal formation, 

 are principally of shells, and of these the most frequent 

 are those which resemble the fresh water species ; such 

 are the different species of mytulites. 



