App. C] Dr. FiTTON on the Strata below the Chalk. 3G9 



APPENDIX C. 



LIST OF HEIGHTS.* 



The following list may be useful to those who examine the country described in the preceding 

 paper, by furnishing some points of known elevation, from which it will be more easy to estimate 

 the surrounding altitudes, than if the entire heights above the sea were to be conjectured. 



The height of the tides upon the English coast is influenced by so many variable circum- 

 stances, that in selecting a zero for a list of heights, some permanent natural or artificial mark, 

 of easy access, is very much to be preferred to the level of the sea, either at high or at low 

 water ; the latter, especially, being objectionable, from the greater difficulty of ascertaining it, 

 and from its being frequently concealed. On this point, I am supported by the opinion of 

 Mr. Lubbock, whose publications on Tides give great weight to his authority. In the subjoined 

 lists, therefore, I should have adopted, as the common point of reference, one of the standard 

 marks hereafter mentioned, had not the heights obtained during the Ordnance Survey, and some 

 others mentioned below, been all referred to the level of the sea at low water ; while, as the 

 mode in which that point was determined is unknown to me, I have thought it best not to make 

 any change in the original measures, by conjectural reduction. 



The Tide-marks on the Thames, in London and its vicinity, which are referred to in legal pro- 

 ceedings, and which form the base line, or " datum ", of many of the railroad sections, have been 

 derived from a long series of observations on the depth of water, first made, I believe, at the 

 East India Docks, by Capt. Huddart, and subsequently at the London Docks ; from which 

 latter place, marks, corresponding to the average spring tide high-water mark, have been 

 transferred by levelling, to most of the bridges, and to the lock on the Thames, at Teddington 

 above Richmond, which is, at present, the farthest range of the tide. The time of high water 

 varies a little, at different points along this interval, but the ultimate heights are supposed to be 

 the same throughout. 



In the year 1830, Mr. Lloyd having fixed a standard mark at the Sheerness dockyards, found 

 the mean rise and fall of the spring tides at that place to be 17*6150 feet; and of neap tides 



* The exact determination of heights is, by geologists, regarded as of small importance ; but the 

 judgment upon this subject is, perhaps, unconsciously influenced by the difficulty which attends it; 

 since if we could measure heights as easily as horizontal distances, it cannot be doubted that the 

 former would be recorded and reasoned upon more frequently. As geology advances, it becomes 

 more and more necessary that none of the elements which enter into the physical history of the globe 

 should be neglected ; and it is probable that many important inferences, not at present foreseen, 

 might result from a correct acquaintance with the levels throughout large portions of the earth's 

 surface. Already it has been ascertained in Sweden, that the land has been gradually raised 

 within a few years, by a small but very perceptible quantity ; and in all countries, especially those 

 subject to earthquakes, it is a point of great interest to determine whether the relative levels 

 of the sea and land, are truly as permanent as they are supposed to be. In the Survey now in 

 progress in Ireland, and in the operations for a new map of France, the heights are carefully 

 ascertained and inserted on the maps : perhaps it may still be hoped, that some general system 

 will be adopted for connecting the measurement of heights in England with the Ordnarice 

 Survey. In the mean time it is satisfactory to find that at the late meeting of the British 

 Association for the Advancement of Science, (August, 1836), a committee was appointed, and a 

 portion of the funds placed at their disposal, (small, it is true, with reference to the object), for 

 the purpose of devising and conducting observations on the relative level of the land and sea 

 upon our coastSi 



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