^*^^- 55-] LIMESTONE-KIS^OLLS IN CRAVET<". 343 



A microscopic slice from the rock at the head of Stockdale, 

 "where silicification has commenced, shows abundant crystals of 

 smoky quartz which often display a zoned structure. The perfection 

 of the crystals suggests a peculiar condition of matrix during the 

 time of their formation, and seems to bear out the observations which 

 I have made concerning the limestones of the knolls, that their 

 crystaJlizatiou was due to removal of pressure, and that they were 

 in a quasi-fluid state before their final consolidation. 



It is interesting to find dolomitizationandsilicification recorded by 

 Mr. E. B. Welhered^ in the limestones of Barton and Lummaton, 

 near Torquay, which are in every respect similar to those of the 

 knolls of Craven, and themselves exhibit knoll-structure. 



One other piece of evidence bearing upon knoll-structure yet 

 remains to be advanced, and that is its artificial production by 

 lateral pressure. The figure facing this page (fig. 8) shows theresult 

 of one of many experiments made by Bailey Willis, and is copied from 

 the 13th Annual Report of the United States Geological Survey 

 (1891-92) pi. xcvi. The knoll is complete, even to the production 

 of brecciatiuu at the margin. 



Y. The Breccias. 



Associated with the reefs are several kinds of breccias, which 

 have undoubtedly been formed in more ways than one. In the tirst 

 place, ordinary fault-broccias occur, consisting of angular fragments 

 of limestone, usually embedded in veined crystalline calcite, such 

 as the breccia seen at the extreme left of the eastern end of 

 Draughton Quarry. These breccias were probably produced at a 

 late period of the movements which have affected the rocks, the 

 l)articalar breccia at Draughton not being anterior to the production 

 ot the sharp- folds. 



Of other breccias one m?„y find three principal varieties : — (i) frag- 

 ments of limestone in a limestone-matrix, the fragments having 

 the same general composition as the matrix; (ii) large nodules of 

 black limestone of irregular form, set in shales of ^vhich the divi- 

 sional planes frequently sweep round the fragments ; (iii) angular 

 and frequentl}"- rounded fragments of various kinds of limestone 

 iu a fine calcareous paste, the rock in some cases resembling a 

 conglomerate, and the fragments and paste usually breaking across 

 with equal ease. I believe that all these varieties have been 

 produced as the direct result of earth-movemeuts, that they are in 

 fact fault-breccias, and I propose to consider each kind in turn. 



Of the first kind I have referred to two examples, one in the 

 Draughton Quarry, p. 333 (see also fig. 9, p. 344), and another at 

 Winterburn (p. 34U;. I have given reasons in each case for supposing 

 that the breccias were produced by mov^ement, in the first iustani'e, 

 as the result of simple fracture of a limestone-band between two 

 others moving along bedding-planes at different speed, and without 



^ Quart. JoLirn. Geol. Soc. vol. xlviii (1892; p. 377. 



