ON SEISMOLOGICAL INVESTIGATION. 113 



Clay has disappeared entirely, as is the case with the Middle Chalk on 

 the north side of the fracture, as well as the Wealden and Upper and 

 Middle Purbeck on the south side. The fault has also become a fracture 

 of unusual sharpness for one of so great a magnitude. 



In discussing the character and extent of thrust of the fault at 

 Ridgeway, it should not be forgotten that it does not pass through a 

 series of conformable strata. The Upper Cretaceous rocks here rest 

 unconformably on a folded and greatly eroded surface of Lower Cretaceous 

 and Jurassic strata, so that the local absence of Wealden and of most of 

 the Purbeck may be due to this unconformity. These intra-Cretaceous 

 folds have an axis approximately parallel with the much later Tertiary 

 disturbances. The most important of them is the wide anticline between 

 Upway and Portland. This is followed northward by a narrow and 

 sharp syncline, which brings in the Wealden and Purbeck between Upton 

 and Bincombe, and passes unconformably under Upper Cretaceous rocks 

 towards the east and towards the north-north-west. Next follows an 

 anticline, which is almost entirely hidden by the newer rocks. It is 

 touched at Poxwell, where the Jurassic strata dip northward at a higher 

 angle than the Upper Cretaceous. It then seems to run beneath the 

 Chalk parallel to the southern boundary just north of the Tertiary over- 

 thrust. Its southern limb reappears at Bincombe, but soon disappears 

 again beneath the overthrust mass of Chalk. The position and character 

 of these earlier folds, their relation to the Upper Cretaceous overlap, and 

 the relation of both to the overlap of the Bagshot Beds on to the Oolite,' 

 are the factors which produced a continuous plane of weakness extending 

 obliquely downward from the surface deep into the Jurassic strata, as 

 shown in the diagram (fig. 3). 



The outcome of this geological structure has been that any subsequent 

 lateral compression in a north and south direction causes the massive 

 Chalk, over 800 feet thick, to be driven against the wide arch of rigid 

 Purbeck and Portland rocks extending towards Portland. Any such 

 movement must tend still more to fold and buckle the already existing 

 small anticlines and synclines ; but the main arch of hard Upper Jurassic 

 rocks would offer great resistance, as v/ould the horizontal ihick-bedded 

 Chalk. Thus the Chalk must approach the main anticline, overriding the 

 minor folds, taking with it such parts of them as happened to be above 

 the plane of greatest weakness, and smearing the slide-plane with Oxford 

 Clay and Cornbrash caught up in the passage over the northern limb of 

 the anticline. 



The above explanation will, I believe, account for the whole of the 

 curious phenomena recorded along this line of fault. Granted north and 

 south compression, any differential movement must be along this plane of 

 weakness. The extent of the differential movement must also be greatest 

 at the surface where the plane emerges, and must rapidly decrease down- 

 ward and northward until the fault entirely disappears. The extent of 

 the movement in this case is probably about half a mile. 



From the data in the memoirs and maps of the Geological Survey, and 

 from my notes made more recently, I have constructed the subjoined 

 geological section across the fault at the point where our apparatus is 

 fixed (fig. 4) ; but though the underground structure must be not unlike 

 that indicated, the exact curve of the fault, and also the exact character 



' See Reid, 'Geology of Dorchester,' chapter vi., Memoirs Gfeol. Survey, 1899. 

 1900. I 



