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Excerpt from Memoirs of the Geological Survey of Great Britain and of the Museum of Practical Geology: The Geology of North Wales <p>The plan of the book is to give, first, a general sketch of the Geology of Wales, and then such a detailed description of the Silurian rocks of North Wales, that any one may ascertain the structure of any minor area in which he may be interested. An index-map and many sections and diagrams accompany the Memoir, but the special knowledge sought to be communicated can only be mastered in some cases by reference to the larger maps of the Geological Survey, published in sheets, on a scale of one inch to a mile, and to the sections, on a scale of six inches to a mile, which illustrate the maps. Nevertheless, for general purposes, the main geological features of N orth Wales may be understood by help of the index-map, sections, and diagrams that accompany this Memoir; and, indeed, by merely reading those parts that are printed in large type all the chief facts connected with the subject may be made out. It is evident that such a book would be ia complete without an Appendix on the Fossils, which was, for the first edition, prepared by Mr. Salter, and has now been corrected and extended by Mr. Etheridge, with that accurate knowledge of Silurian forms for which both have been distinguished. <p>A few words of explanation may here be given with regard to the sense in which the word greenstone is used in this book. It embraces several varieties of rocks in which triclinic felspar forms an ingredient associated with hornblende or augite, and which as originally solidified afterwards sometimes underwent changes both physical and chemical, as, for example, the change from augite into hornblende. Those associated with the Lower Silurian vol canic lavas and ashes have this in common, that all of them are intrusive, and also that their intrusion ranged through various periods of time that extended from the formation of the Arenig rocks to the close of