LECTURES ON GEOLOGY. ^41 



cd of the arrangement of strata, and the mode of their formation, 

 from the labours of Pallas, Saussure, Werner, Hutton, and Smith. 

 The Geological Society, which was established in 1807, having 

 assumed it as their maxim that there were not yet sufficient data 

 for general conclusions, or "Theories of the Earth," put a stop to 

 the violence of the disputes between the Huttonians and Werne- 

 rians, and turned the attention of both parties to support their 

 opinions by the observation of a greater number of facts. 



The Lecturer next exhibited a geological map of England, upon 

 •which he pointed out the various strata ranged in a curvilinear 

 form round the primary mountains of Cumberland and Wales j and 

 explained, by sections in different directions across the kingdom, 

 how the inclined strata, by their '' out crop" at the surface, produce 

 corresponding changes in the appearance of the soil. 



As several groups of these strata, or formations, have a greater 

 degree of relation to each other in position, structure, and con- 

 tents, than to those which follow them in the series, they have 

 been divided by geologists into certain classes. One arrangement 

 makes four great divisions of the strata into primary, transition, 

 secondary, and tertiary ; but as this mode involves an absurdity in 

 making an interval between the primary and secondary rocks, a 

 new classification has been proposed, which taking the coal as the 

 middle of the series, determines the rest by their position with 

 respect to it. Thus the primary corresponds to the inferior, the 

 transition to the submedial, the medial is the coal, the secondary 

 is the supermedial, and the superior is the tertiary class, which last 

 Mr. Lyell again divides into eocene, meiocene, and pleiocene 

 strata, according to the lateness of their deposition. 



Notwithstanding the infinite variety of forms and colours that 

 the rocks, stones, and earths assuntie which compose these strata, 

 almost all of them, except the metallic ores, consist of the follow- 

 ing minerals, silica, alumina, lime, magnesia, potash, and iron 

 oxide. If, therefore, we take granite and marble as examples of 

 the combination of these materials to form primary rocks, we can 

 account for all the newer formations, except the coal, by their 

 disintegration and reconstruction under different forms. Granite 

 consists of quartz, felspar and mica, and marble is merely car- 

 bonate of lime ; and both are easily corroded by the action of 

 the atmosphere and water. The quartz of the former is converted 

 into sand, and the felspar and mica into ferruginous clay > while 

 the marble is reduced to chalk. The claim of the primary rocks to 

 this appellation rests upon their inferior position with regard to the 

 other strata, their crystalline structure, and their not containing 

 any organic remains. By the Wernerians they were considered to 

 form the floor of the earth, upon which the succeeding strata have 

 been deposited j and by the Huttonians, as old materials worked 

 up again into new^ forms by volcanic heat. The transition rocks 

 were so called, from their passing by almost insensible gradations 

 from the crystalline structure of the primary strata, into forms 

 decidedly indicating a mechanical origin by subsidence from water j 



