OIL SHALES— SCOTTISH CARBONIFEROUS SYSTEM. 245 



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 lepidodendro7i and spheiwpteris, and is at places directly 



