270 Martinis Geologicul Memoir^ and 



The contemporiety of these acts being established, it follows, 

 of course, that the chalk, with all its superincumbent strata, 

 existed previously in a horizontal position." [Id., p. 89.) 



The true disposition of a chalk basin, according to Mr. 

 Martin's diagram {Jig, 134.), arises from the fracture and sub- 

 sidence of its parts ; for, although large hollows have doubtless 

 been formed upon its surface, its thickness is not adequate to 

 admit of the construction of a basin in that way only. 



*''^^^~^~~px^^ 



From the foregoing extracts an accordance will be per- 

 ceived, between these authorities, in the conviction that, an- 

 terior to the great derangements of the Weald and the chalk 

 basins, the strata had existed in a horizontal position, and that 

 '' what are called tertiary formations (those above the chalk) 

 were deposited before the basins were in existence," and 

 covered a far wider area than they have done subsequently to 

 the deluge. " A wide field is thus opened, not for conjecture, 

 but for research ; and, if the subject of derangement be culti- 

 vated with the same fidelity of observation as has been of late 

 that of structure, results cannot fail of being produced, de- 

 structive of many errors, and corrective of many received 

 opinions." (Martin, p. 90.) 



In a note, appended to Mr. Martin's memoir, some con- 

 jectures are offered on the probability of lines of fracture tra- 

 versing the London clay, over which the Thames is conducted, 

 and on the presumed risk to which the construction of a 

 tunnel would hence be subjected. The direction of these fis- 

 sures would resemble those in Mr. Martin's diagram. [Jig. IS^.) 

 *' A tunnel through the London clay, in those parts where the 

 river has been directed over it, in an unsuhsided or undisturbed 

 state, is perfectly practicable ; but, where rent and subsidence, 

 or what are commonly called faults, have taken place, there 

 remains nothing but loose diluvial and alluvial soil, through 

 which it must be at all times dangerous to venture. If, there- 

 fore, the expedients for repairing the mischief fail in this 

 instance, a spot might still be chosen where the river takes its 

 course over the clay, that has not been disturbed by the convid- 

 sion *which basined the chalk" (Martin, p. 89.) 



