34 Notices of Memoirs — On Strike-Majis. 



(d) StratigrapJiical. — Much detailed work is required in many 

 parts of the world to discover over what periods coal-formation 

 occurred in exceptional amount. Tendency at outset to refer all 

 Upper Palaeozoic coal-formations to the Coal-measures. 



IIL — On the Construction and Uses of Stkike-Maps.^ By 

 J. LoMAS, A.R.C.S., F.G.S. 



IN studying the deformations which a series of rocks have undergone, 

 we are apt to regard the vertical movements as all-important, 

 and neglect the horizontal movements to which they have been 

 subjected. This is largely owing to the difficulties experienced 

 in picturing such horizontal movements and representing them 

 on a plan. Lines dependent on surface inequalities confuse the 

 worker when he seeks to use the ordinary geological maps for this 

 purpose. It is easy to get rid of these lines by projecting the 

 strikes of the beds on to a horizontal plane. We then have the 

 appearance that would be produced if the country were planed down 

 to a horizontal surface. The outcrops would coincide with the 

 strikes, and any deviation from straight lines would indicate 

 horizontal movements. Vertical movements would also be shown 

 on such a plan by the closing up of outcrops of beds of equal 

 thickness. All the data necessary to represent these features 

 on a strike-map are given in the ordinary Geological Survey 

 sheets. To construct such a map, first trace the dips given 

 on the geological map and draw short lines at the points of the 

 arrows, at right angles to the direction of dip. We thus have 

 represented the strikes of the beds at a number of points. Now 

 it is necessary'- to connect these up by lines to show the strike at 

 intermediate places. It would not be safe to connect one line with 

 another, as the strikes may refer to different beds. In order to 

 overcome this difficulty, draw a series of lines parallel to the strike 

 line on both sides of it. On doing this for all the positions it will 

 be found that the lines either connect themselves in linear series, 

 or we have represented a series of tangents to curves which become 

 evident when the lines are prolonged in the direction of the strike. 

 Care should be taken not to connect in the same line strikes with 

 dips in contrary directions, and it is well to represent the dip side of 

 the strike lines by a short mark i . When the amount of 



dip is known, as well as the direction, we can represent the 

 steepness of the folds by suitable shading, either by hachures or 

 closeness of strike lines. As an illustration I exhibit strike maps 

 of the district about Clitheroe, including the well-known knolls 

 at Worsa and Gerna. The anticlinal ridge just north of Chatburn 

 is clearly shown, and the strata dipping with wavy folds towards 

 the Kibble on the north and Clitheroe on the south. The Salt Hill 

 quarries are excavated in this southern slope at a place where the 

 fold becomes acute. The knolls at Worsa and Gerna appear like 

 whirls or eddies, such as may be seen in a stream when the flow is 

 obstructed by boulders in the stream bed. 

 1 Eead before the Eritish Association, Section C (Geology), Bradford, Sept., 1900. 



