28 The Ash Bed at Meriden. 



disturbed position. The lengths of the books should be imagined 

 of indefinite extension. Let pages 90 to 100 of every book repre- 

 sent the heavy trap sheet of the second eruption; then lift the left 

 hand side of every book, corresponding to western side of the blocks, 

 so that the book covers will slope fifteen degrees to the east. In 

 order to give a closer imitation of the real occurrence, the books 

 should lie with the dividing lines between them trending to the 

 northeast, and should then be lifted at the western corners, so that 

 the direction of their slope is eastward and not at right angles to 

 the lines of dislocation by which they are separated. The uneven 

 surface thus produced corresponds to the constructional form that 

 the Triassic formation would have had when first disturbed, if no 

 erosive forces had worked upon it during its disturbance. Now 

 imagine the books worn down to a certain level, corresponding to 

 the base-level of the first cycle of erosive work; and on this level 

 surface, pages 90 to 100 of all the books would have several outcrops. 

 It is further manifest that on such a base-levelled surface the sequence 

 of pages can be found in proper order only by following a line that 

 crosses the pages of a single book; in other words, the true sequence 

 of Triassic deposits can be found only by crossing the country in a 

 line between the faults that l)ound any given block; otherwise, in 

 passing from one block to another, the line would traverse beds out 

 of their normal order; and a single bed might be met as often as 

 the line passed into a new block. On this geometrical principle 

 depends the method that must be employed in unravelling the Tri- 

 assic structure; the observer must keep between a pair of faults that 

 enclose a block, if he would not confuse his section by encounter- 

 ing the members of the series out of their proper order. 



It is simply impossible to understand the structure of the region 

 and the problem that it presents without a clear comprehension of 

 its geometrical relations, somewhat after the manner given here. 



Now after the elevation of the old base-level plain, the erosive 

 forces begin a new cycle of destructive work upon it; they wear 

 down the sandstones rather quickly, but the trap sheets are more 

 obdurate and withstand the weather more successfully. This is es- 

 pecially true of the heavy main sheet, which has not as yet lost much 

 of the form that it had in the late stages of the history of the old 

 plain, although it has gained relief by the lowering of the adjacent 

 country. Wherever there was an outcrop of the main trap sheet 

 on the old plain, there we now have a ridge developed; and inas- 

 much as the tilting and dislocation of the blocks into which the 

 mass has been divided had produced repeated outcrops of the 



