268 STRATIGRAPHICAL GEOLOGY 



fossils ; but the Wardie shales contain Schizodus pentlandicus, 

 Naiadites obesa, Anthracomya Icevis, and others. These beds 

 gradually thin westward till near Glasgow there is only 100 feet 

 of such sandstone. 



Oil - shale Group. This group takes its name from the 

 prevalence of black bituminous shales which contain large 

 quantities of petroleum, some bands of them being so rich as to 

 yield from 30 to 40 gallons of oil per ton of shale, besides a 

 considerable quantity of sulphate of ammonia as a by-product in 

 the process of abstraction. 



Near Edinburgh there is a bed of oil-shale above the Hailes sand- 

 stone which makes a convenient base to the group, and about 900 

 feet above this is the Burdiehouse limestone, a bed of dark-grey 

 limestone 20 to 30 feet thick and crowded with the cases of the 

 small Ostracod Leperditia Okeni (var. burdigalensis). Another lime- 

 stone band higher up is entirely composed of the shells of 

 Productus semireticulatus and P. longispinus. 



The group is most fully developed in Fifeshire, where it is over 

 3000 feet thick, containing many seams of oil-shale and no fewer 

 than eighteen beds of limestone containing marine fossils ; but the 

 mass of the group consists mainly of shales and sandstones which 

 contain the remains of fish, plants, and estuarine molluscs, especially 

 Naiadites obesa. These sediments are interstratified with sheets of 

 volcanic material basalts and basaltic tuft's and are pierced by a 

 large number of pipes and bosses of volcanic agglomerate which are 

 the orifices of small volcanic vents. 



When followed westward by Stirling and Dumbarton to Renfrew 

 and the Isle of Arran, the oil-shales become much thinner and are 

 interstratified with lava-flows, which in some districts seem entirely 

 to take the place of the sedimentary deposits. 



Lower Limestone Group. Near Edinburgh this group 

 has a thickness of 400 feet to 500 feet, and contains three beds 

 of limestones, but these are seldom more than 12 feet thick, 

 though occasionally swelling out to 30 feet, and in the Bathgate 

 Hills (Linlithgow) to 70 or 80 feet. In Ayrshire the Hurlet, or 

 Main limestone, attains a thickness of 100 feet. Spirifer trigonalis, 

 Productus giganteus, and P. semireticulatus are common fossils, and 

 in some places the limestones are largely crinoidal, i.e. composed 

 of the shelly plates which form the arms and stems of crinoids. 

 These limestones are interbedded with a series of shales, sandstones, 

 and coal-seams, several of the coals being immediately succeeded 

 by a limestone, e.g. the Hurlet coal and limestone, as if there had 

 been times when the shallow lagoons and swamps were suddenly 

 submerged and occupied by marine organisms. 



