XXV11 



DESCRIPTION OF THE MAP. 



The accompanying Map is intended to convey a clear general idea of the geological divisions of 

 the surface of the tract illustrated, the details being explained by numerous coloured sections. 

 It is a reduction of the Ordnance Survey, and therefore exhibits accurately the drainage and the 

 relative positions of the towns and villages : the form of the hills is not delineated, but as many of 

 the heights are marked by figures, and the names of the ridges are inserted in the line of their di- 

 rection, it is hoped that the general features of the country will be understood. 



In the selection of colours, I have represented each great system of strata under one common 

 tone, and its sub -formations by modified tints of the same colour. The calcareous rocks of all 

 ages appear under shades of blue or green; the formations to which they are subordinate being 

 known by the colour which surrounds them. The rocks of igneous origin are all represented by 

 bright red, a method which I have employed for some years 1 , from having thought that masses, the 

 varieties of which pass imperceptibly into each other, ought at once to strike upon the eye as 

 being derived from the same source. But after all, as colours are fugitive, and details of high 

 interest are often crowded together into small spaces, I have inserted letters at intervals (par- 

 ticularly on the sectional lines), the meaning of which is at once explained by reference to the 

 table of superposition. 



In addition to the tabular arrangement, usually employed by other authors, I have placed below 

 the map on the right hand, a general transverse section, in which the sedimentary formations 

 and their subdivisions are represented in the order in which they naturally overlie each other, 

 where no volcanic rocks are present ; whilst on the left, an ideal section is given of the submarine 

 condition of these sediments before their elevation from beneath the sea, to mark the periods when 

 volcanic action was most in play, either when giving rise to fine detritus of scoriae and ashes, accu- 

 mulated in alternating layers with sand and mud ; or during the emission of those intrusive rocks, 

 which, whether syenites, greenstones, porphyries or basalts, are all the products of fire. Many of 

 these volcanic rocks being inseparably connected, even within small areas, and in the same ridges, 

 their lithological distinctions can only be understood by consulting the explanations in the text ; but 

 the epochs at which they are supposed to have been erupted, are signified by different letters, — their 

 relations to the sedimentary deposits being rendered more intelligible by reference to the plates of 

 coloured sections (see Plates 29 — 37). 



A small map of England and Wales has been engraved in the corner of the large map, for the 

 double purpose of pointing out the geographical site of the Silurian Region, and of subdividing the 

 kingdom geologically into eight sedimentary groups, each of which I conceive (in British classifi- 



1 In the Map of the Eastern Alps, Geol. Trans., vol. iii. PL 35., and in the coloured Ordnance Maps of 

 the Silurian Region exhibited at various meetings of the British Association, and at the Geological Society 

 from 1832 to 1837. I am glad to perceive that Mr. Griffiths has adopted this method in the new Geological 

 Map of Ireland. 



d 2 



