518 Notices of lUeinoIrs — Ptij)rrs read before Section C. 



Ordovician) lavas of the Southern Uplands. Pillow-lavas with kerato- 

 phyres, etc., characterize this group. They are not connected with 

 movements either of Atlantic or of Paoific kind, and may be pla:'<&d in 

 a special family. 



XI.— On Buckled Folding.^ By G. Barrow. 



A NUMBER of descriptions have been published of portions of areas 

 of regional crystalline metamorphism in which the dip of the 

 bedding, or in some cases the dijj of the foliation, is described as being 

 at a low angle over a considerable area. These descriptions are at 

 times so worded as to convey the impression that this represents the original 

 and but slightly disturbed bedding of the altered sediments. Experience 

 is gradually j)roving that the altered sediments in such areas are always 

 intensely folded, and the detailed examination of the Highlands suggests 

 that in place of these long-continued low dips being due to small 

 disturbance, thej'^. really represent the most complicated form of structure, 

 for which the name of ' buckled folding ' is suggested. 



The best-known illustration of the jihenomenon in this country is 

 afforded by the gneissose-flagstones or Moine gneisses. The deceptive 

 nature of the dips in the rocks was soon recognized bj'' the officers of 

 the Geological Survey of Scotland, who found that in clifp sections the 

 beds really ascended the cliffs by a zigzag course to which the term 

 'lightning-structure' was applied. 



The mode -of production of this 'zigzag' structure can be traced on 

 the cliffs between Stonehaven and Muchalls, in Kincardineshire. The 

 rocks in these cliffs consist of alternations of grits, gritty shales, and 

 shales, becoming more and more crystalline as we proceed northward. 

 Nearer Stonehaven the bands of grit may be seen to ascend the cliffs 

 in an unbroken or unbent course from bottom to top, having a high 

 dip in a northerly direction. The limbs of the folds are thus isoclinal 

 and unbent. "But as we jiroceed northwards the course of the grit bands 

 up the cliff face is no longer straight ; a small overf old or ' buckle ' 

 is developed on^ it. At first only one is seen in the whole height of 

 the cliff ; further north two occur ; then three, and so on, till at last 

 they are so close together that the still straight portion of the fold is 

 little, if at all, longer than the 'buckled' or overfolded portion. If 

 the upward course in the cliff of each grit band be carefully followed 

 it will be found that the oncoming of this ' buckling ' structure does 

 not alter the dip of the "band as a whole ; it still descends at much 

 the same angle, only it pursues a more zigzag course. The overfolds 

 or buckles all face the same direction right up to Muchalls, and a little 

 consideration will show that this ' buckling structure ' must have been 

 produced after the isoclinal folding was completed. There is thus no 

 evidence to suggest that the rocks in which the buckling structure has 

 been developed should be separated as a different scries from those in 

 which it does not occur. In the interior of the South-Eastern Highlands 

 this structure is present in all but the southern margin of the area, 

 till we reach the first outcrop of the Highland quartzite, where the 

 buckling rapidly ends and the earlier isoclinal folding is deft unaltered. 

 For a distance of some seven or eight miles the buckling structure is 

 either very rare or absent, but it sets in again in the quartzite close to- 

 Braemar. It is there also shown both by the marginal sandy beds, 

 known as the Moine gneisses, and the dark schist next them. There also 

 is no justification for sei^arating the beds with the superinduced 

 ' buckling structure ' from those without it, as has been generally done. 

 A key to the connexion between the ' buckled ' and ' non-buckled ' areas 

 occurs in the ground about Shiehallion and to the north (Sheet 55, 

 Scotland). At Shiehallion we have the quartzose beds forming the 



^ Abstract of paper read before the British Association, Section C (Geology), 

 Dundee, September, 1912. 



