Editing Instructions for the Local Directors of the Geological Survey of Great Britain and Ireland Henry de la Beche 22nd May 1845

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It is essential to study these in connection with the rocks of sedimentary origin, including among the latter, for convenience, the various limestones, that as close an approximation as possible may be made to their true geological dates in addition to the ordinary methods of investigation; more particularly seeing how far they may generally resemble the volcanic products of the present day and how far other conditions may be necessary to account for the Phenomena observed. Though apparently so simple there are few geological subjects which have been more fruitful in erroneous conclusions from the want of careful investigation than this. The consolidated ashes of subaerial eruptions of the earlier epochs sometimes resemble solid trappean rocks, or those which have been ejected in a state of igneous fusion, as to have been frequently mistaken for the latter. During the progress of the Geological Survey of Great Britain instances have occurred where bands of organic remains have been discovered in rocks which at first sight appeared as resulting from complete igneous fusion, and specimens obtained by unskilled persons, striking off fragments only of the non-fossiliferous portions would have been so termed, leading to erroneous geological conclusions. The history, if the term may be employed, of the intermingling of the igneous with the sedimentary rocks is most important, and often very complex, so that the Geologists and Assistant Geologists employed cannot be too strongly cautioned carefully to study it, requesting aid when difficulties arise, as indeed they should be directed most especially to do in all cases when they may consider it desirable, inasmuch as over confidence on the one side, and expectation of perfect fitness on the other would be most detrimental to the service. In studying masses of igneous rock it is important to observe the effects produced by the different rates of cooling upon the same chemical compounds, varieties of rocks from simply compact to highly crystalline being frequently the results of the different conditions to which the mass has been exposed, often known by different names, though really formed of the same amount of the same elementary bodies. Great care is required not only in observing these phenomena, so important geologically, but also in selecting such specimens for examination in the Laboratory, should that be desirable, as shall fully illustrate the subject. Unskilful research in this respect would be a lamentable waste of the opportunities afforded by a geological Survey of the character of that of the United Kingdom. The subject requires investigation on the large scale, bearing in mind that a map several square miles in extent is but minute considered geologically. It is by carefully investigating points of this order that we arrive at the relative fusibility of the masses, can draw fair inferences respecting their original composition, and reason properly upon the changes they are supposed often to have effected on the sedimentary rocks with which they have been brought into contact, either when thrust up amid or overflowing them. It is often very desirable to ascertain that this contact has not been such as to melt up some portion of the sedimentary rocks adjacent to the original molten mass, thus adding to it, and in such a manner, that the really igneous product may appear to shade off into sedimentary rocks, containing organic remains, the intermediate states being, as it is termed, metamorphic crystals, such as those of feldspar, appearing, and giving such rocks a porphyritic character. It would be needless to advert to a variety of other points connected with igneous rocks, to which attention has long been called and which are well understood by Geologists.  
 
It is essential to study these in connection with the rocks of sedimentary origin, including among the latter, for convenience, the various limestones, that as close an approximation as possible may be made to their true geological dates in addition to the ordinary methods of investigation; more particularly seeing how far they may generally resemble the volcanic products of the present day and how far other conditions may be necessary to account for the Phenomena observed. Though apparently so simple there are few geological subjects which have been more fruitful in erroneous conclusions from the want of careful investigation than this. The consolidated ashes of subaerial eruptions of the earlier epochs sometimes resemble solid trappean rocks, or those which have been ejected in a state of igneous fusion, as to have been frequently mistaken for the latter. During the progress of the Geological Survey of Great Britain instances have occurred where bands of organic remains have been discovered in rocks which at first sight appeared as resulting from complete igneous fusion, and specimens obtained by unskilled persons, striking off fragments only of the non-fossiliferous portions would have been so termed, leading to erroneous geological conclusions. The history, if the term may be employed, of the intermingling of the igneous with the sedimentary rocks is most important, and often very complex, so that the Geologists and Assistant Geologists employed cannot be too strongly cautioned carefully to study it, requesting aid when difficulties arise, as indeed they should be directed most especially to do in all cases when they may consider it desirable, inasmuch as over confidence on the one side, and expectation of perfect fitness on the other would be most detrimental to the service. In studying masses of igneous rock it is important to observe the effects produced by the different rates of cooling upon the same chemical compounds, varieties of rocks from simply compact to highly crystalline being frequently the results of the different conditions to which the mass has been exposed, often known by different names, though really formed of the same amount of the same elementary bodies. Great care is required not only in observing these phenomena, so important geologically, but also in selecting such specimens for examination in the Laboratory, should that be desirable, as shall fully illustrate the subject. Unskilful research in this respect would be a lamentable waste of the opportunities afforded by a geological Survey of the character of that of the United Kingdom. The subject requires investigation on the large scale, bearing in mind that a map several square miles in extent is but minute considered geologically. It is by carefully investigating points of this order that we arrive at the relative fusibility of the masses, can draw fair inferences respecting their original composition, and reason properly upon the changes they are supposed often to have effected on the sedimentary rocks with which they have been brought into contact, either when thrust up amid or overflowing them. It is often very desirable to ascertain that this contact has not been such as to melt up some portion of the sedimentary rocks adjacent to the original molten mass, thus adding to it, and in such a manner, that the really igneous product may appear to shade off into sedimentary rocks, containing organic remains, the intermediate states being, as it is termed, metamorphic crystals, such as those of feldspar, appearing, and giving such rocks a porphyritic character. It would be needless to advert to a variety of other points connected with igneous rocks, to which attention has long been called and which are well understood by Geologists.  
  
=== Sedimentary Deposits  ===
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=== Sedimentary Deposits. ===
  
 
Investigations on this head should be so conducted that the probable manner in which such rocks have been accumulated should be well understood. The mere grouping together of a number of beds, collectively known by a particular name, finding the boundaries of the mass, and inserting these on the maps and sections is insufficient in the present state of Geology. The different conglomerates, sandstones, slates, shales, marls, &c have to be studied with reference to their original condition of variously distributed gravels, sands, silt, clay, mud, &c. This is highly important, more particularly when taken in connection with any organic remains which may be found in them, for if it be considered that physical changes in the bottoms of seas have produced modifications in the animal life inhabiting them, it follows that particular animal remains, once considered characteristic of given sedimentary rocks, may rise higher in the series, or have existed lower than is supposed, when similar accumulations of sand, silt, mud, or otherwise as the case may be, have occurred over a particular area for corresponding geological periods.  
 
Investigations on this head should be so conducted that the probable manner in which such rocks have been accumulated should be well understood. The mere grouping together of a number of beds, collectively known by a particular name, finding the boundaries of the mass, and inserting these on the maps and sections is insufficient in the present state of Geology. The different conglomerates, sandstones, slates, shales, marls, &c have to be studied with reference to their original condition of variously distributed gravels, sands, silt, clay, mud, &c. This is highly important, more particularly when taken in connection with any organic remains which may be found in them, for if it be considered that physical changes in the bottoms of seas have produced modifications in the animal life inhabiting them, it follows that particular animal remains, once considered characteristic of given sedimentary rocks, may rise higher in the series, or have existed lower than is supposed, when similar accumulations of sand, silt, mud, or otherwise as the case may be, have occurred over a particular area for corresponding geological periods.  

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