136 STRATIGRAPHICAL GEOLOGY 



Arenig Series. From this table it will be seen that so far as 

 the beds are exposed the greater part consists of volcanic rocks, but 

 that the highest member is a group of mudstones and shales inter- 

 stratified with lenticular layers of red and grey chert which are 

 full of the remains of Radiolaria. From this fact it has been inferred 

 that these beds were oceanic deposits formed under a great depth 

 of water and at a great distance from land. Elsewhere, however, I 

 have shown that such an inference cannot be drawn from the 

 mere occurrence of Radiolaria (see Building of the British Isles, 

 3rd edition (1911), pp. 12 and 72). 



The narrow compass, within which the beds representing the 

 Ordovician System are compressed in the Moffat district, is shown 

 in Fig. 42, which is reduced from a section in the Survey Memoir. 



Fig. 42. SECTION THROUGH TROWDALE IN THE URR VALLEY. 



(Scale about 750 feet to an inch.) 



Silurian/ 6 ' Gala Beds - OrHovir-ian j 4 - Hartfell shales. 2, 1. Chert Beds and 



man t 5. Birkhill shales. aovician-j 3 Glenkiln shales Volcanic tufts. 



In the Barr Series the Stinchar limestone is an interesting 

 horizon, for many of its fossils are not found anywhere in England, 

 but occur in the Trenton limestone of North America ; such are 

 Maclurea Logani, M. magia, Murchisonia angustata, the sponge- 

 like coral Tetradium, and Saccamina Garteri ; there are also many 

 corals (Lyopora favosa, etc.) and brachiopods, with Lichas sexcostatus ? 

 Illcenus latus, I. Bowmanni, and Calymene Blumenbachi, most of 

 which range from Upper Llandilo to Bala Beds. 



The Balclatchie Beds are included in the Barr Series by Professor 

 Lapworth, but he speaks of them as forming a transitional band 

 or zone, and many of the graptolites range into the Hartfell shales. 

 Messrs. Peach and Home place these beds in the Ardmillan or 

 Bala Series, but leave the Benan conglomerate in the Barr Series. 

 It is possible that the plane of divisions should be drawn between 

 the Balclatchie shales and the overlying grits. 



The Ardmillan Series, which is the equivalent of the Welsh 

 Bala Beds, consists of a great thickness of alternating flagstones, 



