438 S. AUport — The Pifchstones of Arran. 



separating the Upper and Middle Coal-field may be reduced in 

 thickness towards Wigan, and still further in the district under con- 

 sideration, as it is kaown that the Middle Coal-measures lose about 

 one-third of their thickness between Wigan and Prescot (Vertical 

 Sections of the Geol. Survey, Sheet 61). 



It is possible, then, that future exploration may prove the existence 

 at no great depth of equivalents of the Bradford and Clayton Coal- 

 seams in the measures which underlie the Trias, north of the 

 positions of the bore-holes where the limestone was proved, but 

 south of and overlying the 1100 feet of unproductive strata met 

 with in the Bold Hall and Collins Green Collieries. Whether the 

 seams, if they exist, are likely to prove sufficiently valuable to repay 

 the cost of sinking to them must remain for the present a matter of 

 speculation. The observation that the attenuation of the measures 

 referred to above as taking place between Wigan and Prescot is 

 accompanied by a deterioration in the quality of the coal in the same 

 direction is not in favour of their doing so. In the absence of good 

 coal in these beds, the great mass of unproductive measures that 

 would have to be penetrated to reach the productive Middle Coal- 

 measures, when added to the thickness of overlying Trias, will 

 prevent the Triassic areas bordering the Mersey from becoming a 

 repaying coal-field, at least until the more accessible fields have 

 been more nearly exhausted than at present. 



II. — Note on the Pitchstones of Aeran. 

 By S. Allpoet, F.G.S. 



IN 1872 I published in the Geological Magazine (Vol. IX. p. 1), 

 a brief account of the Arran Pitchstones, in which I figured and 

 described the singularly beautiful groups of minute green crystals 

 which are so characteristic of the rocks on the eastern shore of the 

 island. I have since cut many additional thin slices, and have long 

 been aware that my paper contains an error, which I will take the 

 present opportunity to rectify. The groups of crystallites to which 

 I refer are described on page 2 and illustrated, Plate I. Fig. 1, and 

 contain the " long slender prisms of a green pyroxenic mineral " 

 there mentioned. In the last paragraph, p. 9, these green prisms 

 are again mentioned, and are said to be augite, instead of hornblende, 

 as stated by Zirkel in his description of the same rock. The grounds 

 for this conclusion were twofold, the absence of dichroism, and the 

 presence of undoubted crystals of augite. After pointing out that 

 the pale green crystals exhibit no trace of dichroism, I added, "Unless, 

 therefore, it can be shown that some hornblende is not in the least 

 dichroic. the mineral in question must, on this ground alone, be 

 regai-ded as augite." ' As regards the latter mineral, there can be 

 no doubt whatever of its occurrence, for in addition to the crystal 

 described in the paragraph just quoted, I may now add, that it is 

 far from uncommon, perfectly characteristic examples having been 

 observed by me in specimens from several different localities in the 

 island. Having obtained on the former occasion clear proof of the 

 presence of augite, I have now to admit, that there is equally strong 



' Op, cit. p. 9. 



