development of new minerals, crushing out and stretching of fossils and fragments and, in short, for any structure which is familiar to him as a result of orogenic movements. [Footnote 21: Lamplugh, G. W., "On the Crush-Conglomerates of the Isle of Man," _Quart. Journ. Geol. Soc._, vol. LI. p. 563.] [Illustration: Fig. 5.] The effects of thrusting not only give rise to appearances suggestive of unconformity, but naturally also to a simulation of overlap. The thrust-planes are often parallel to original bedding-planes for some distance, but must cut across them sooner or later, producing lenticular masses which might be supposed to be due to the thinning out of beds as the result of cessation of deposition in a lateral direction. Attention has already been directed to the deceptive appearance of great thickness of strata which is due to repetition of one stratum or set of strata by a series of thrust-planes, so that there is no actual inversion of any part of a bed. When masses of limestone are affected in this way, the thrust-planes may become sealed up, as the result of chemical change, and a compact irregular mass of limestone devoid of any definite divisional planes may be the consequence, and beds of grit sometimes exhibit the same feature to some extent. Enough has been said to show that simulation of one structure by another has frequently occurred in rocks in so marked a degree as to render mistakes easy; and that these examples of 'mimicry' in the inorganic world are particularly frequent in rocks which have been subjected to great orogenic movements. The student will do well to acquaint himself with the macroscopic and microscopic structures which may be taken as characteristic of the rocks which have been thus affected, some of which can usually be detected with ease, and when he discovers them he may suspect that many phenomena which appear explicable in one way were in reality produced in a different one, for it is frequently very true of a region in which the rocks have been violently squeezed, stretched and broken that 'things are not what they seem.' CHAPTER VIII. GEOLOGICAL MAPS AND SECTIONS. The writer does not propose to give an account of the intricacies of geological mapping, for their right consideration requires a separate treatise[22]; all he desires is to call attention to some of the uses of geological maps as a means of conveying information. A geological map may be looked upon as an attempt to express as far as possible in two dimensions phenomena which possess three dimensions; this can be done to some extent on the actual surface of the map, by conventional signs, still more fully, by supplementing the map with sections; but best of all by a geological model, which is cut across in various directions in order to show the underground structure as well as that of the surface.