I. GEOLOGY. 817 



The gradual advance of English geology (as expressed by maps) is shown 

 to some extent by the successive editions of the Index of Colours used by the 

 Geological Survey. 



The earliest index (in MS. only) was drawn up by Sir Henry De la Beche, 

 and shows the classification of rocks in use, and the extent to which geological 

 surveying was carried in the year 1832. 



In the first published index the volcanic rocks were indicated by green 

 colours, but Sir R. I. Murchison, on becoming director-general in 1855, intro- 

 duced the sytem of lettering the various formations, beginning with A for 

 the Cambrian rocks, and altered the colouring of the igneous rocks to various 

 shades of red. The results of these changes are shown on the second sheet. 



In the index of colours published in 1856, the system of colouring, lettering, 

 &c. then introduced will be seen to be essentially the same as that now in use. 

 Various modifications have, however, since been made from time to time, for 

 the purpose of representing the more numerous subdivisions of certain for- 

 mations which it has become necessary to map, and some alterations have also 

 been made in that part of the index which relates to the igneous and meta- 

 morphic rocks, so that the nomenclature of the maps of the Survey may 

 approximate more closely to that of modern petrology. 



3252b. Geological Map of Cornwall and West Devon. 

 Ten Sheets on the scale of one inch to one mile, mounted together. 



In this map the granitic masses of the S.W. of England are shown partly 

 surrounded by Devonian rocks and partly by rocks of Carboniferous age, the 

 latter overlying the Devonian rocks conformably, and filling up part of a 

 synclinal trough, so that the Devonian rocks occupy superficial areas north 

 and south of that composed of the Carboniferous rocks (culm-series). Dykes 

 of Elvan (quartz-porphyry), which proceed from the underlying granite, are 

 seen to trend in an east and west direction, while interbedded greenstones 

 (rocks of a doleritic type, including beds of volcanic ash) follow the strike 

 of the Devonian and Carboniferous rocks in various places. Masses of ser- 

 pentine and diallage rock are likewise marked on the map, and also the tin, 

 copper, iron, and lead lodes which occur in different localities, the stream-tin 

 deposits being also indicated. 



This map has a historical interest, since it is almost entirely the work of Sir 

 Henry De la Beche, the founder and first Director of the Geological Survey of 

 the United Kingdom. The publication of this map and the accompanying 

 report on the geology of Cornwall, Devon, and West Somerset led to the 

 recognition of the scientific and economic value and importance of a geological 

 map of the country ; to the institution of a Government geological survey 

 under Colonel Colby, R.E., who was then Director of the Ordnance Survey ; 

 and afterwards to the foundation of the Museum of Economic Geology (now 

 the Museum of Practical Geology), to illustrate the work of the Survey, and 

 its practical application in mining, agriculture, architecture, and the fine arts. 

 The Royal School of Mines was subsequently added, to afford facilities for 

 the teaching of geology and the collateral sciences. 



3252c. Geological Map of East Devon, Somerset, 

 Dorset, and part of Wilts. (Scale, one inch to one mile.) 



Ten sheets, on the scale of 1 inch to a mile, mounted together, illustrating 

 not less than 53 formations in East Devon, Somerset, Dorset, and part of 

 Wilts. The map includes the various tertiary beds of East Dorset, the chalk 

 of Salisbury Plain and of Dorsetshire, including that of Ballard Down, together 

 with the S.W. range of the Upper Greensand of Shaftesbury and-Westbury, 

 and to the west that of Black Down and Great Haldon. 



40075. 3 F 



