l6 INTRODUCTION. 



L. Jack has pointed out, do not lose their continuity.' The CIt:at 

 of a coal-seam, is a term applied to interrupted layers of coal sepa- 

 rated by shales, and may be caused by fracture. 



When one formation succeeds another, and extends over its 

 margin, this is called Overlap, and is no doubt due to the sinking 

 of the area of deposition. A good example of this occurs in the 

 INIendip country, where the Lias overlaps the Penarth Beds. (See 

 Fig. 24.) Where, however, one formation stretches over the out- 

 crop of a series of strata, this is an unconformable overlap or 

 ovasiep: and this is the case where the Chalk and Greensand 

 stretch across the outcrops of the Lower Secondary strata in Dorset 

 and Devon. 



The strata are sometimes disturbed and bent into folds : the 

 basins or troughs of these undulations are known as synclinals, the 

 saddles or ridges are anticUnals. When the strata are very rapidly 

 and irregularly folded, they are said to be cotiloried. 



The Pennine anticlinal, the Mendip anticlinal and the Weymouth 

 anticlinal are familiar exhibitions of this kind of structure. The 

 INIerioneth anticlinal, described by Sedgwick, brings up Arenig, 

 Tremadoc, and older rocks on the south-east side of Snowdon, 

 while the fossiliferous Bala strata of that mountain occupy a 

 synclinal.- The structure of the London Basin is also an example 

 of a synclinal. 



Sometimes beds are found to be actually folded over or inverted. 

 On the northern side of the Mendip Hills the Coal-measures are 

 so much disturbed, faulted, and contorted that the same coal-seam 

 has been penetrated three times in one shaft. The most interest- 

 ing features in connection wdth these disturbances are the little 

 masses of Carboniferous Limestone which at Luckington and 

 Vobster occur in the midst of the Coal-district, and beneath which 

 in places coal has been worked. The position of these masses 

 seems to be due to a complicated system of inversion and faulting. 

 Prof. Phillips has pointed out that in the Abberley district there 

 is an anticlinal of Aymestry rock which has the singular character 

 of being folded or bent on an axal plane dipping to the east, so 

 that the Ludlow rocks overlie the Old Red beds, while on the east 

 the ridge of Wenlock Limestone is seen dipping in the same 

 direction, as if it were a superior stratum.^ 



The rocks are divided into numerous groups, and systems, 

 and minor subdivisions, all of which, when looked at in a 

 large way, are seen to be merely terms of convenience, for the 

 ancient history of the earth, so far as we know it, like the 

 ' Short History of Our Own Times,' shows a sequence of 

 events uninterrupted by any universal physical break. And 



1 G. Mag. 1S69, p. 505, 1 87 1, p. 388. 



2 See Phil. Mag. (4), viii. 125 ; and A. C. Ramsay, Geology of North Wales, 

 Edit. 2, Plate 28, Sec. 3. See also Figs, 20, 21, 24, etc., of the present work. 



^ Mem. Geol. Survey, Vol. ii. Part I. p. 151. 



