Reports and Proceedings — Geological Society of London. 279 



and siliceous inylonite, the latter being next to the Torridonian 

 rocks and lying along the line of a great post-Cambrian thrust, 

 traceable from Oronsay to near the Point of Sleat. On the eastern 

 side of the mylonite all the rocks of sedimentary aspect are 

 greatly altered, and the gneisses and schists associated with the 

 limestone resemble parts of the Lewisian gneiss. The quartzite- 

 series is repeatedly folded with the gneissose series ; but although 

 the precise relationship of the two has not been made out, rocks 

 of different characters lie next to the quartzite in different places, 

 while the alteration-products which characterize the limestone are 

 found in contact with the quartzite at one or two spots. In the 

 limestone, pieces of diopside and a serpentine-like mineral are so 

 abundant that they may equal the rest of the rock in quantity. 

 The mass of the limestone is but little affected by movement, thus 

 offering a strong contrast to the Loch Maree limestone. The 

 forsterite and spinel are in part associated together in lumps, from 

 which they were separated by means of heavy solutions for analysis. 

 The spinel in hand-specimens is of an almost opaque blue colour, 

 and some examples show small crystal -faces. That seen in micro- 

 scopic slides is shapeless and colourless, except that the blue 

 portions appear brown by transmitted light. Analyses of the two 

 minerals are given at the close^of the paper ; and it is pointed out 

 that the spinel is like that of Aker in colour and mode of occurrence. 



III.— April 26, 1899.— W. Whitaker, B.A., F.E.S., President, in 

 the Chair. The following communications were read : — 



1. " On Limestone-Knolls in the Craven District of Yorkshire 

 and elsewhere." By J. E. Marr, Esq., M.A., F.R.S., F.G.S. 



The author begins with a general account of the district, partly 

 founded on the published work of Mr. R. H. Tiddeman, but 

 substantiated by his own observations. The Lower Carboniferous 

 rocks north of the Craven Fault-system differ in character and thick- 

 ness from those on the south ; they exhibit little disturbance on the 

 north, but on the south they are thrown into a series of folds, while 

 it is also on this side that the knobs of limestone called knoll-reefs 

 by Mr. Tiddeman occur. In order to illustrate the nature of the 

 disturbances south of the Craven Faults, the folds of Draughton 

 Quarry are described in detail. In addition to the anticlines and 

 syncline usually figured, there occurs, on the north side of the 

 quarry, an overfolded anticline with a faulted core. One of the 

 calcareous bands is really a limestone-breccia, which appears to have 

 been broken by earth-movement before the rocks were folded. The 

 top and base of the breccia and of other hard bands, together with 

 the joint-faces, are traversed by slickensides which were horizontal 

 before the folding. Microscopic examination of the limestones shows 

 that they exhibit various stages in crushing with development of 

 a ribboned structure due to movement, while the matrix in other 

 ■varieties has become crystalline, as though under relief from pressure. 



