The Mosses and Hcpatics of Denbighshire. 2S7 



Millstone Grit and the hills of Cyrn y brain, lies a patch of 

 rock which is supposed to be Old Red Sandstone, together 

 with grey sandstones and cornstones, concretions containing 

 some limestone. It contains no fossils, so that its age is 

 rather uncertain. 



4. The Denbighshire Grits 



are represented in the Llangollen District. These consist of 

 an extensive series of shales, flagstones, sandstone and grits, 

 and are marked by the absence of limestone. They are of 

 Lower Wenlock and LTpper Silurian age. They cover an ex- 

 tensive tract of country, mostly moorland, in which the Heather 

 association is dominant — Dicranum scoparium, Campylopus 

 fiexuosus, C. Pyrijormis, Rhacomitrium fasciculare, Alicularia 

 scalaris, Gymnocolea inflata and Lophozia ventricosa. 



5. Ordovician or Lower Silurian. 



These comprise the imposing hill called Cyrn y brain, 

 which lies to the west of the Carboniferous Limestone. It 

 is surrounded by wettish heathery moorland. The commoner 

 mosses and hepatics are well distributed over this area and 

 grow in quantities, especially those belonging to the genera 

 Sphagnum, Polytrichnm, Campylopus, Rhacomitrium and 

 Grimmia. In Merionethshire, Grimmia arenaria is generally 

 associated with Coscinodon cribrosus and Grimmia Stirtoni on 

 this formation, but hitherto it has not been met with in the 

 district, although the last two mosses occur. The other species 

 characteristic of the flora of this ground are: — Ceratodon pur- 

 pureus, Webera nutans, Bryum pseudo-triqitctritm, B. capillarc 

 Brachythecium populeum, Plagiothccium clcgans, P. denti- 

 culatum, Scapania compacta, Alicularia scalaris, Lophozia 

 quinqucdentata, Marsupdla emarginata and Diplophyllum 

 albicans. 



The corticole species are rare in the general district referred 

 to, on account of the atmospheric pollution by noxious products 

 resulting from the incomplete combustion of coal. These 

 products are discharged from the chimneys of coal mines and 

 steel works, as well as from numerous domestic fires. Mr. 

 J. A. Wheldon, in his excellent paper on the Lichens of South 

 Lancashire, states that the burning of coal is accompanied by 

 the liberation of sulphur dioxide (SO..), which becomes oxidised 

 in the atmosphere into sulphuric acid (HoSO,), and brought 

 down in the rain. Such sulphur compounds as sulphur di- 

 oxide and sulphuretted hydrogen produce injurious effects on 

 vegetation. He also states that soot has a very deleterious 

 effect on most arboreal plant growths. As rain is carried down 

 the tree trunks, it becomes more and more charged with acid 

 impurities, which must prove fatal to young and tender plants. 

 A film of soot forming on the surface of the bark must also 



1917 Sept. 1. 



