I. GEOLOGY. 691 



The rocks composing the great anticlinal ridge of the Mendip Hills mark 

 the boundary of the Somersetshire coal-field, which here is generally concealed 

 by Triassic, Liassic, and Oolitic overlying formations ; when, however, the 

 coal measures are exposed, the coal seams are indicated by continuous black 

 lines ; these lines, when broken, show the extension of the seams beneath the 

 overlying deposits. In the S.W. portion of the map the Devonian rocks of 

 Totnes, Torquay, and Start Bay are shown, and also the Carboniferous and 

 Devonian rocks of Mid Devon, and the easternmost portion of the granite of 

 Dartmoor. 



3252d. Geological Map of the Wealden Area. 



In addition to the Wealden area, this map comprises the southern and main 

 parts of the London Tertiary Basin and the eastern portion, of the Hampshire 

 Basin, which are separated from each other by the intervening parallel range 

 of chalk hills, commonly known as the North and South Downs respectively. 



The upper and lower cretaceous rocks, from the Upper Greensand to the 

 Lower Greensand inclusive, appear in succession from beneath the chalk, 

 forming an outer margin surrounding the Wealden area, which is separated 

 from the central portion, consisting of the alternations of sand and clays 

 (called the Hastings Beds), by the Weald clay, a broad belt of low and gene- 

 rally flat ground. 



The model constructed by Messrs. Topley and Jordan (No. 3248) will convey 

 a better notion to the eye of the general configuration of the ground than can 

 be afforded by the map. 



The latter, however, shows the undulations of the chalk, which forms large 

 synclinal and anticlinal folds or elevations and depressions, and the enormous 

 amount of cretaceous strata which once extended over the space between the 

 North and South Downs, and which must have been removed by denudation. 



3252e. Geological Map of North Wales, on the scale of 

 one inch to a mile. 



The rocks consist of the Cambrian and Lower Silurian strata, from the 

 bottom of the Menevian beds to the top of the Bala beds, including the Bala 

 limestone, with associated eruptive rocks and contemporaneous lavas and 

 ashes, also of Upper Silurian strata of the Wenlock series, and of Old Red 

 Sandstone, Carboniferous Limestone, Coal Measures, and Permian beds. 



In Anglesea, the Cambrian rocks consist of grits and gneiss, and the 

 Silurian strata of the island are also partly gneissic and associated with granite 

 rocks. In Carnarvonshire, the Cambrian rocks largely consist of grits and 

 purple slates, which are extensively quarried for slate, especially at Llanberis 

 and Penryhn. In Merionethshire, the same formations consist chiefly of 

 massive purple and green grits, with a little purple slate. The contempo- 

 raneous volcanic series occurs at the top of the Arenig, and also in the Bala 

 series, the top of Snowden being a calcareous volcanic ash contemporaneous 

 with the Bala limestone. Most of this series is fossiliferous. 



The Upper Silurian beds consist of the Denbighshire flag-stones and Wen- 

 lock shales, also fossiliferous. 



The Old Red Sandstone (unfossiliferous) is sparingly developed in Anglesea 

 and Denbighshire. The Carboniferous Limestone and overlying mill-stone 

 grit and Coal Measures are shown in Anglesea, Denbighshire, and Flintshire, 

 and these are in places succeeded by red Permian strata and by the New Red 

 series. 



3252f. Geological Map of Mid Wales, on the scale of one 

 inch to a mile. 



Xx 2 



