Along the western escarpment of the basaltic mountains in Londonderry, the mulattoe is seen underlying the chalk in Kedy Donald and Ballyness hills. Between Ballyness and Benbradagh, at the head of the vale of the Kelvin river, a wild circus opens, barred on the north by basaltic ridges, and having the insulated primitive district of Coolcroscrahan on the south: the Glen Ullin water and Donavenny brook rise also within the area, and flowing in a direction contrary to that of the Kelvin, join the Aghivey river itself, tributary to the Bann. Within this area a remarkable bed of marle occurs, which is mentioned here, on account of its containing numerous green particles; but it differs so essentially from the Mulattoe in its chemical composition, containing only one-fiftieth of calcareous matter, that it cannot with propriety, though resembling it in external appearance, be considered as the same rock: the base is a soft clay, greyish, with a slight red tinge; the structure is thin slaty; it contains some small worn quartz pebbles: the chalk has not yet been traced in this district, but along the banks of Donavenny brook, a breccia formed of chalk flints, imbedded in a calcareous cement, and underlying the basalt, seems to indicate its proximity. This breccia is probably connected with the aggregation of flints, usually interposed between the basalt and chalk, as described in the next article.
The subjoined list of the organic remains occurring in the Mulattoe of Ireland, will be found to agree closely with those presented by the green sand in England.
