TRANSACTIONS OF SECTION C. 743 



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

 strike line ou 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 ' 



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 VVorsa and Gerna. The anticlinal ridge just north of 

 Oliatburn 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. 



4. On Eapid Changes in the Thickness and Character of the Coal Measures 

 of North Staffordshire. By W. Gibson, F.G.S. 



[Communicated by permission of the Director-General of H M. Geological SurVey.] 



Variability in thickness and character of the strata is universal throughout the 

 Carboniferous period, but is nowhere more marked in the Midlands than in the coal- 

 field of the North Statibrdsbire Potteries 



This important coalfield consists of two portions. On the east the productive 

 measures lie in a well-marked syncline, while on the west the strata rise in a sharp 

 anticline extending from Silverdale to Talke. The two productive areas are 

 separated by a strip of ground two and a half miles broad, composed of barren 

 upper measures. 



A notable difference in the thickness of the strata and nature of the coal seams 

 characterises these structurally distinct areas. In the centre of the syncline, near 

 Shelton, the vertical distance between the highest ironstone, or summit of the 

 productive measures, to the BuUhurst coal, or lowest workable seam, is about 

 1,.300 yards. On the anticline at Apedale only 800 j'ards of strata separate the 

 same horizons. This makes a remarkable decrease in tliickness of 500 yards of 

 strata in a distance of under three miles. The reduction in thickness westward of 

 the productive measures is continued, though in a less degree, in the upper barren 

 series, but owing to the absence of shaft sections the amount cannot be definitely 

 statsd It is known, however, that the red marls forming the lower portion of 

 the upper barren j-eries are more than 1,000 feet thick near Etruria station on the 

 Shelton property, and about 850 leet thick near Silverdale, on the south-eastern 

 limb of the anticline. With the decrease in thickness a change has takt-n place in 

 the lower coals of the productive series. The seams which are house or steam coals 

 on the east change into gas and coking coals on the west. 



This great variability seems to show that separate areas of deposit were being 

 marked out by local movements of elevation and depression, and thus fulfilling iu 

 North Staffordshire the conditions characteristic of the Carboniferous of the 

 Midlands generally, as pointed out by Prof. Lapworth.^ 



In North Statlordshire it happens that the areas of maximum and minimum 

 deposit correspond with a syncline and anticline. If this is true generally, and not 

 merely a local coincidence, we may expect the coals in the unexplored coalfield 

 which lies at the surface to the west of the anticline, and which represents the 



' A Sketch of the Geology of the Jiirmingham, District {Geol. Assoc), 1898, 

 p. 364. 



