Page:The Journal of geology (IA journalofgeology21894univers).pdf/259
traversed by numerous necks, sheets and dykes of intrusive igneous rock, to map which, even with the aid of copious mining information, is often a task of the greatest difficulty. The rapid thinning and thickening of some of the members of the series also adds an element of difficulty to the elucidation of the structure of the district, and although many hundreds and even thousands of borings have been made in search of oil shale and other rocks, there are places where, in the absence of surface exposures, the structure of the area has still to be ascertained.
The total thickness of the Carboniferous Limestone series of Mid and West Lothian is about 2,000 feet, and that of the upper or oil shale division of the underlying calciferous series is a little over 3,000 feet. Beneath the oil shale series there is a group of shales, fire clays and gray sandstones, one of which, formerly quarried very extensively at Craigleith, supplied nearly all the celebrated freestone from which a large part of the New Town of Edinburgh was built. Owing to its great hardness and cost of working the Craigleith stone has now been nearly given up and is only used where great strength is wanted in house architecture, or for export to America, where it appears to be used occasionally for special work. This middle division of the Calciferous Sandstone series appears to be about 3,000 feet thick, and the red beds which lie below cannot be much thinner, so that 9,000 feet may be taken as the approximate distance from the top of the Old Red Sandstone to the highest part of the Oil shale series in this locality.
Dealing now in greater detail with the Oil shale group of rocks, it may be noted, first, that the whole series has a fresh water or estuarine character, and contains few or no strata of a marine type. There are numerous thin bands of concretionary unfossiliferous limestone, and one well-marked bed of richly fossiliferous estuarine limestone of workable thickness. This rock—the Burdiehouse Limestone—in addition to fresh water shells, contains fish plates, teeth, etc., and many plant remains such as lepidodendron and sphenopteris, and is at places directly