THK ORDOVICIAN SYSTKM 131 



rocks of this system (1832-36). All the mountainous region 

 lying north-west of a line drawn from the estuary of the Duddon 

 by Hawkshead and Ambleside to the valley of the Lowther Beck 

 consists essentially of Ordovician rocks (see the map, Fig. 39, and 

 section, Fig. 37). 



They are also found in a long narrow inlier, known as the 

 Cross Fell inlier, on the eastern side of the Vale of Eden in the 

 north-east of Westmoreland, bounded on the one side by Carboni- 

 ferous limestone and on the other by New Red Sandstone. 



The Ordovician rocks of this area are divisible into four 

 . which are roughly equivalent to those of Wales. 



Skidd avian Series. It was stated on p. 93 that the greater 

 part of the great mass of slates which have generally been known 

 as the Skiddaw slates was of Ordovician age, but as the highest 

 portions belong to the Llanvirnian Series it is only the central 

 portion which is strictly the equivalent of the Welsh Arenig Series, 

 and for this part the name Skiddavian may be used. Its basement 

 beds are certain beds of grit which are called the Watch Hill grits 

 in the memoir of the Geological Survey on the country between 

 Appleby, Ulleswater, and Haweswater (1897). 



Above this horizon the slates have yielded both trilobites and 

 graptolites, which have enabled Dr. J. E. Marr and Miss G. L. 

 Elles 14 to recognise three zonal divisions which, however, cannot at 

 present be correlated exactly with those of Wales. They are : 



3. Upper Tetragraptus Beds, yielding Tetragraptui serra, Didymograptus 



indentus (var. nanus), and PhyUograptus typus. 



2. Dichograptus Beds, Dichograptus 8-branchiatus, and Did. patulus. 

 1. Lower Tetragraptus Beds, with Tetr. Bigsbyi. 



The whole series is so greatly flexured and faulted that it is 

 impossible to form any accurate estimate of its thickness. 



Llanvirn Series. This is typically developed in the 

 Cross Fell area, where the two well-known components of the 

 series have been distinguished under the names of Ellergill and 

 Milburn Beds respectively ; the former being black slates containing 

 Didymograptus bifidus and Glossograptus armatus, the hitter being 

 mainly volcanic rocks (ashes and lavas) interstratified with slates 

 ^vlli(ll have yielded Did. Murchisoni and Diplograptus dentaius. 



In the Lake District beds belonging to the lower stage occur 

 near Threlkald and Troutbeck, east of Keswick, and those of the 

 upper stage near Ulleswater and elsewhere. 



Llandilo Series. This consists of an immense thickness of 

 volcanic rocks rhyolitic and andesitic lavas with thick beds of 

 a<h and agglomerate (see map, Fig. 39), which are generally of 



