414 CONSISTENCY OF GEOLOGY WITH SACRED HISTORY. 



the " Results" of a course of detailed and exact induction, involving 

 extensive and precise knowledge of several collateral sciences, and 

 especially of conchology, botany, and comparative anatomy. 



The chapter, although detached from its documents, is mainly in- 

 telligible without them, and presents a fine example of the course of 

 induction now pursued by the most able geologists and is precisely in 

 point in support of our present argument. Mr. Mantell, remarks : — 



Happily, the evidence of the great physical mutations, and impor- 

 tant changes in organic life, which have taken place in this part of the 

 earth during the geological periods to which our researches refer, is 

 so clear and satisfactory, that even the general reader will perceive 

 that our deductions, extraordinary as they may appear, naturally re- 

 sult from the facts themselves. 



The several formations or groups of strata, previously described, 

 may be regarded as geological chronometers, marking certain distinct 

 epochs or periods ; the lowermost or most ancient of which (as we 

 have already noticed) is of fluviatile origin, and reposes on the Oolite^ 

 a marine formation of great extent, that forms an important feature in 

 the physical structure, not only of England, but also of the Continent. 

 The Portland Limestone constitutes the uppermost division of the 

 Oolite, and contains marine remains only ; it is succeeded by the 

 fresh-water strata of the Isle of Purheck, which may be considered 

 as the lowermost deposits of the Wealden. 



But there is a fact connected with the history of the Portland and 

 Purbeck beds, so highly interesting, and which illustrates in so stri- 

 king a manner the nature of one of those grand geological mutations 

 which have taken place in the south of England, that it will be neces- 

 sary to notice it here, although it occurs without the limits of the dis- 

 trict, which it is the professed object of this work to describe. 



In the island of Portland, the oolitic limestone is extensively quar- 

 ried for architectural purposes, and supplies most of the cities and 

 towns in the south-east of England. On these oolitic strata are pla- 

 ced deposits of a totally different character. Immediately on the up- 

 permost marine stratum (which abounds in ammonites, terebrcs, tri- 

 gonia, &LC.) is a bed of limestone, much resembling, in appearance, 

 some of the tertiary lacustrine limestones. Upon this stratum is a 

 layer of what appears to have been an ancient vegetable soil ; it is 

 of a dark brown colour, contains a large proportion of earthly lig- 

 nite, and, like the modern soil on the surface of the island, many 

 water-worn stones. This layer is called the dirt-bed by the quarry- 

 men ; and in, and upon it, are a great number of silicified trunks of 

 coniferous trees, and plants allied to the recent cycas and zamia. 

 Many of the stems of the trees, as well as the plants, are still erect, as 

 if petrified while growing undisturbed in their native forest ; the for- 

 mer, having their roots in the soil, and their trunks extending into the 

 superincumbent strata of limestone. On a late visit to the quarries, a 

 large area of the surface of the dirt-bed having been cleared, prepara- 

 tory to its removal for the purpose of extracting the building-stone 

 from beneath, several stems, from two to three feet in height, were 



