306 G. W. Tyrrell — Bekinkinite of Barshaw, Renfrewshire. 



amount of rock visible is too small for this fact to have any distribu- 

 tional significance. 



In the western corner of the quarry an exposure of decomposed 

 rock shows a band of hard, grey, spotted material (variety No. 7), 

 and three lenticular streaks of the leucocratic variety mentioned above. 



The Barshaw exposure forms part of a persistent intrusion which 

 appears near the Hurlet horizon in the Paisley district, and has been 

 named the Hurlet sill. 1 A fine exposure occurs in the railway 

 cutting at Arkleston, another at the Blackhall Quarry, and the River 

 Cart near by. Exposures also occur at Seedhills and Old Crookston 

 Farm. All these localities lie to the east of Paisley, and have been 

 fully described by P. Macnair. 2 A sill is also seen in the old quarry 

 at the Fossil Grove, Victoria Place, Whiteinch, on the north side of 

 the Clyde. 3 According to the Geological Survey this exposure 

 belongs to the Hosie sill, a small intrusion from 24 to 30 feet in 

 thickness, which occurs near the Hosie Limestone horizon, about 

 20 fathoms above the Hurlet sill. 4 Mr. Macnair thinks it highly 

 probable that the Seedhills, Blackhall, Arkleston, and Whiteinch 

 intrusions belong to the same great mass and are connected under- 

 ground. Mr. B,. G. Carruthers also, in the Glasgow Memoir, thinks 

 it probable that the Hosie sill is an offshoot from the Hurlet. 



At Arkleston and Blackhall the Hurlet sill has become involved 

 with the Hurlet Coal, and has suffered great endomorphic meta- 

 morphism, resulting in that modification the extreme term of which 

 is known as 'white trap'. This mode of alteration is general 

 throughout the mass of the sill, except perhaps in the centre. It is 

 consequently exceedingly difficult to trace any resemblance between 

 these rocks and the Barshaw theralites. If the Barshaw exposure 

 belongs to the Hurlet sill, it has ascended or descended in this area 

 to a horizon devoid of carbonaceous material, and has escaped the 

 ' white trap ' mode of alteration. 



The determination of the true petrographic character of the Hurlet 

 sill (apart from the Barshaw exposure) is much hindered by its 

 extensive decomposition. The freshest material occurs at Old 

 Crookston Farm and at Hurlet. The latter rock is described in the 

 Geological Survey Memoir on the Glasgow District (p. 133) as an 

 ophitic dolerite with purple augite, abundant olivine, and but little 

 analcite. It is stated to resemble somewhat the Gallaston type of 

 Fife. This description well fits the rock from Old Crookston Farm. 

 My specimen shows numerous areas of chlorite rosettes which 

 doubtless obscure areas of analcite, as they do in other undoubted 

 analcite rocks from the same region. Areas of calcite also occur in 

 polygonal spaces between the felspars in such a way as to suggest 

 that they have replaced analcite. Alkali-felspar (probably soda- 

 orthoclase) may also occasionally be detected in small quantity. The 

 rock from the centre of the Arkleston sill is of the same general 

 character, but is finer-grained, and its augite has a very deep purplish 

 tint. These rocks are very similar to the types termed alkali-dolerite 



1 Geology of the Glasgow District (Mem. Geol. Surv.), 1911, p. 116. 



2 Trans. Glasgow Geol. Soc, vol. xiii, pt. i, pp. 58-60, 1907. 



3 Eutley, Q.J.G.S., vol. xlv, pp. 626-32, 1889. * Glasgow Memoir, p. 117. 



