﻿530 MR. E. G. CARRUTHERS ON THE [NoV. I9IO, 



be briefly outlined, the leading datum-lines among the limestones 

 being pointed out. No reference will be made to the Liddesdale 

 development of the Carboniferous Limestone Series, the material 

 collected therefrom being quite insignificant in amount, although it 

 is satisfactory as far as it goes. 



IV. General Succession in the Carboniferous Limestone 

 Series (Central Valley of Scotland). 



Upper Limestone Group. — The strata are largely arena- 

 ceous, the whole thickness varying from 500 to 900 feet (Ayrshire 

 excepted). Pour to five seams of limestone, or calcareous shale, are 

 found, dividing the sequence into approximately equal parts (see 

 PI. XXXVI, fig. 1). These limestones are, in descending order: — 



(4) Castlecary Limestone. 

 (3) Oalray Limestone. 

 (2) Extra Limestone. 



(In the West of Scotland two limestones are found hereabouts ; 



the upper is known as the Orchard, and the lower as the Lyon- 



cross Limestone.) 

 (1) Index Limestone. 



The Calmy Limestone (3) can beyond doubt be traced throughout 

 the Central Valley, from the west coast to the east ; the other 

 limestones can also be followed nearly as far. Our knowledge of 

 the distribution of these beds is unusually good, on account of the 

 innumerable bores put down to reach the coals below. 



Quarry-sections are few, the limestones being, as a rule, of little 

 economic value. 



Lower Limestone Group. — The total thickness varies from 

 100 to 400 feet, the strata being, on the whole, argillaceous. In 

 the western half of the Central Valley there are two marked lime- 

 stone horizons, which define the upper and lower limits of the group. 

 These are, in descending order : — 



(2) Hosie Limestones (one to four seams, all close together). 

 (1) Hurlet Limestone and coal. 



These two horizons can be followed throughout the West of 

 Scotland, their outcrop round the west and south sides of the 

 Lanarkshire Coal-Pasin being traceable for upwards of 54 miles. 



In the East of Scotland the seams have not yet been definitely 

 recognized, owing to the abrupt change that takes place in the 

 character of the limestones near Bathgate, in the heart of the 

 Central Valley. Nevertheless, the general facies in the East of 

 Scotland is similar to that in the West, the strata consisting largely 

 of shaly material with several thin limestone-bands. On general 

 stratigraphical grounds, it appears highly probable that the whole 

 group was continuously deposited throughout the Central Valley. 

 Any homotaxial difference between the eastern and the western 

 facies must be comparatively slight, if, indeed, it exists at all. 



