Whilst the greatest effort has been made to ensure the quality of this text, due to the historical nature of this content, in some rare cases there may be minor issues with legibility. There is probably no branch of geology in which an unaided student has more difficulty in making a start than in the microscopic study of rocks. This is especially to be regretted, as it is now recognised that this method of research is indispensable for any accurate work on the igneous rocks. Many a resident on the great volcanic plateaux of India, Africa, or America, or on the vast tracts of gneiss and schists which form such a large proportion of the earth’s crust, is debarred from geological work by the difficulty of identifying minerals under the microscope. Thus our knowledge of many of these areas is very deficient, for they are only known by occasional rapid traverses; and the problems they present are so complex that they can only be fully worked out by a resident or by a thorough survey.
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