LECTURES ON GEOLOGY. SM 



and carbon as distinct principles ; in his opinion^ all the bitumens, 

 from naptha to asphaltum, consist of compounds of carbon and 

 hydrogen. In the most fluid, the hydrogen predominates and 

 diminishes progressively, according to their relative tenacity or 

 solidity, where asphaltum ends the series of bitumens j cannel coal 

 begins that of the coals, and from cannel coal to the most perfect 

 anthracite, or to plumbago, the hydrogen is less abundant as the 

 coal is less inflammable, and yields a smaller quantity of bitumen 

 by distillation. Anthracite yields no bitumen, though it contains 

 hydrogen, until it passes into plumbago, which is devoid of that 

 element. The vegetable origin of coal appear to be established by 

 its association with strata, abounding in vegetable impressions, by 

 its close resemblance to lignite or fossil wood, and by the fact, that 

 some true coal is composed of layers of carbonized vegetable 

 matter. 



Here the Lecturer exhibited a series of specimens, arranged 

 according to their chemical constituents, from plumbago through 

 the bitumens to naptha on one hand, and from plumbago through 

 coal, lignite, and jet, to peat on the other. 



Many geologists, with Sir James Hall, have believed that the 

 conversion of fossil wood into coal was effected by heat, but the 

 almost total absence of porcelain jasper in contact with the coal 

 beds, and the facility with which the bituminous shale is accounted 

 for by the mere infiltration of bitumen into the adjoining clay, 

 not to mention many other arguments which Dr. Maculloch has 

 adduced, will induce us to suppose that water, and not fire, was the 

 medium of the change from wood to coal. 



The fossil plants found in the strata alternating with the coal 

 beds, are chiefly allied to the fir, cactus, reed, and fern tribes j but 

 fossil botany has made too little progress at present to enable us to 

 decide with accuracy the place of more than a very few of them in 

 the system of plants. 



Shells are rarely seen in the coal itself, but they are very common 

 in the ironstone nodules of the shale. The most abundant genus 

 is the unio, a fresh-water shell, but pectines, producti, and even 

 nautili, are occasionally met with, which are all considered to have 

 been inhabitants of the ocean. The radius of a balistes also bein^ 

 found sometimes in the same strata, gives support to the idea that 

 the coal fields were estuaries, or marshes, occasionally overflowed 

 by the sea. The coal fish which are often shewn in collections and 

 museums belong to the strata superior to the coal, chiefly to the 

 lias ; and their carbonised bodies should be classed with the 

 lignites, and not with true coal. 



The hypothesis that at present prevails among geologists re- 

 specting the origin of the coal strata, is that they are analogous to 

 modern deposits of peat, and that they were formed from the 

 remains of vegetables growing in and around a lake, marsh, or 

 estuary, which was subject to occasional violent inundations. 

 By these floods, much gravel, sand, and mud, together with trees, 

 and plants, and mineral substances in solution or suspension, was 



