340 ME. J. E. MARK ON LIMESTONE-KNOLLS IN [Aug. 1 899, 



they consist of repetitions of hard bands of limestone situated 

 between shales, the repetition being sometimes due to folding 

 accompanied by faulting, and sometimes to faulting alone. In the 

 summer of 1897 an almost circular knoll, about 4 feet in diameter, 

 was exhibited at the eastern end of the Hambleton Rock Quarry at 

 Bolton Abbey Station : it is clearly the result of the thickening out 

 of a single band of limestone. In the Otterburn Valley, east of 

 Long Preston Station, a number of knolls of small size may be seen 

 in plan and section, and in some cases traced to the expansion of 

 thin beds of limestone. There are many other localities where 

 similar phenomena occur, and these small knolls, like the large knolls, 

 are often associated with breccias. In some of the shales surround- 

 ing the small knolls of the Otterburn Valley, the rock breaks up 

 along divisional planes at a high angle to the bedding, the planes 

 are close together and parallel one to another, and I cannot distin- 

 guish the structure from cleavage. 



Pig. 6. — Folded (?J, faulted^ and hrecciated limestone-hand^ 

 Winterburn Reservoir. 



[The above diagram is an attempt to show one band only. Length of 

 section = a few feet.j 



The structure cannot be well brought out by photography, and 

 drawings are of little use, as the draughtsman will probably delineate 

 more than he sees. It is difficult, therefore, to convince anyone 

 who has not inspected the district of the repetition of beds, for 

 generally simple overfolding is absent, and thrusting is the rule ; 

 nevertheless the termination of lenticular masses of limestone in 

 loops against a divisional plane is sufficient to indicate where repeti- 

 tion has occurred, and this structure is frequent. To explain what 

 I mean, I reproduce here from my note-book a section of part of a 

 knoll of white crystalline limestone, exposed in making the reservoir 

 at Winterburn (and now, alas ! submerged), which was shown to me 

 by Mr. Tiddeman in the year 1888 (fig. 6). This I take to be a 

 case of repetition of a thick band of limestone upon itself several 

 times, the limestone becoming brecciated on the right of the section, 

 but it is only fair to state that some who saw the section would not 

 admit this explanation, and Prof. J. Walther, who admitted the 

 folding, attributed it to drag of the edge of a reef. Very significant 

 also is the appended section (fig. 7), taken from an exposure of 

 limestone several yards high in the Otterburn Valley, near Long 

 Preston, which shows what is almost certainly a case of pseudo- 

 stromatism. (I may here observe that in many sections, where the 



