Vol. 55.] LIMESTONE-KNOLLS IN CRIVEN. 337 



18 inches thick. The major part of the limestone is obviously com- 

 posed of fossils, but the uppermost few inches possess a marked 

 parallel structure, and the fossils appear to be rolled out, while at 

 the base the parallel structure is more marked and no fossils are 

 visible to the naked eye. The same remark applies to the chert- 

 band limestone. 



A microscopic examination of the base of this rock, and also of 

 the base of the limestone containing the chert-band, reveals appear- 

 ances which generally resemble those exhibited by the thin limestone 

 that occurs in the middle shale. The greater part of each rock 

 consists of limestone with a structure resembling f^ow-structure; 

 where this has not been very fully developed the rock exhibits 

 a brecciated appearance, and the matrix has a parallel structure, 

 the lines of discontinuity ' flowing ' around the brecciated fragments. 

 The shale-bands themselves show signs of stretching and crushing, 

 although no fossils were observed in them. Elsewhere I have found 

 similar shales containing Posidonomya and other fossils which show 

 signs of stretching, and are teased out at the edges. 



The phenomena described in the foregoing pages — the breccia- 

 tion, slickensiding, rolling out, and stretching — are all explicable 

 on the supposition that there was differential movement in the beds 

 which at first caused them to slide one over another, the move- 

 ments being in general parallel to the original bedding of the rock. 

 Following these movements came those which caused the sharp 

 folding and minor overthrusts of the rocks, and as the result of 

 this phenomena of considerable interest are exhibited. 



In the left limb of the right fold, the breccia is seen to be 

 thrust into the upper shale, with much disturbance of the latter, 

 though the hard beds above pass over the arch of the fold un- 

 broken, the shale having been squeezed out against the breccia and 

 collected in the hollow below. In the trough of the central syncline, 

 two thin beds of shale above the upper shale have also undergone 

 much movement, being squeezed into a hollow in the trough, and 

 thin hard beds in them are folded and broken. But the most 

 interesting feature is shown in the case of the middle shale, in the 

 arch of the left fold. The limestones above and below have been 

 stretched, and are traversed by radial and tangential veins, while 

 the shale with its central thin limestone has been squeezed away 

 from the sides of the fold, so that the limestone is in places absent 

 there, and the shales are very thin. At the arch the shale is 

 puckered up and thickened, and there seem to be several bands of 

 limestone, which on examination prove to be the folded and broken 

 portions of one (see fig. 4). On the right-hand side of this fold, 

 about 4 feet below the crest of the arch, a remarkable hollow in the 

 limestone (possibly caused by solution under pressure) is filled with 

 arched-up shale ; the thin central limestone is repeated thrice on 

 itself by faulting ; and fragments of it occur detached from the main 

 mass (fig. 5). I call special attention to this occurrence, because it 

 constitutes a minute representation of knoll-structure, showing all 



Q. J. G. S, ^^0. 219. z 



