1897.] NEW YORK ACADEMY OF SCIENCES. 237 



Neglecting thin coals, the section shows about forty feet of 

 coal Ij'ing in a dozen seams. Of these, the Arley Mine is most 

 valuable and has the greatest development, outcropping around 

 the whole coal field. When surveying the Burnley coal field 

 prior to 1874, Professor Hull calculated the Arley Mine to have 

 an area of about 23 square miles. Allowing 5,000 tons per acre, 

 he estimated the total yield as 73,600,000 tons, of which about 

 one-tenth had been extracted, leaving, after deduction for loss 

 and waste, 65,000,000 tons to be mined in the future.* The total 

 yield to 1874 of the whole coal field was estimated at 18,500,000 

 tons, leaving 89,000,000 tons to be mined (Op. cit. p. 83). 



The basin-like area of this coal field causes the mines lying 

 above the Arley to have a much diminished superficial area, so 

 much so that Professor Hull calculates that the Gannister Mine, 

 which passes under the whole of the Middle Measures may yet 

 be made to yield 100,000,000 tons, or more than the whole of the 

 seams of the Middle Series. 



Upper Coal Measures. 



These measures are better developed in the Manchester area, 

 than in any other part of England. The development is, how- 

 ever, altogether local, the other areas of Upper Coal Measures in 

 Lancashire being of insignificant proportions. 



A small patch of shales and flaggy sandstones in the Wigan 

 area, overlying a coal supposed to be the Worsley Four Feet, 

 belong probably to the lower part of the upper series. 



Another small patch occupies the southern border of the 

 south Lancashire coal field in the neighborhood of Leigh, Wors- 

 ley and Pendleton. A portion of the same Measures forms a 

 similar border to the middle series from Kingley to Prestwich, 

 but has been carried to the northwards by the great Irwell 

 valley fault. 



The Upper Coal Measures along the southern border are par- 

 tially concealed by the overlap of Permian and Trias. Since 

 they are mainly unproductive they have not been exploited. 

 They consist of reddish shales, clays and sandstones with thin 

 bands of limestone and a calcareous hiematite worked at Patri- 

 croft. They also contain a coal known as the Yard Coal of Pen- 

 dleton. 



Manchester Goal Field. 



This small coal field has already been mentioned as one in 

 which the Middle Coal Measui-es are still untouched, the rocks 

 nearest the surface belonging solely to the upper series. 



* Memoirs Geolog. Survey. " Geology of the Burnley Coal field," 1875, p. 76. 



