THE STRUCTURE AND CLASSIFICATION OF ROCKS. 



633 



slate and limestone, from which it differs remarkably in composition. Dr. Macculloch has 

 noticed here the metamorphic change, to which reference has been made, produced in 

 the sedimentary strata in immediate contact with the igneous rocks, by their high 

 temperature when obtruded ; for the limestone is changed in character by the proximity 

 of the granitic mass or its veins, and acquires a more compact texture, like that of 

 hornstone, or chert, with a splintery fracture. Besides intruding into the stratified rocks 

 in the form of veins, the granite and other igneous masses are frequently traversed 

 themselves by veins, which show successive epochs in which the fiery fluid from beneath 

 has been erupted, the rock itself thus veined having of course been ejected at the earliest 

 epoch. In one remarkable example of veins of different kinds, near Salem in Massachusetts, 

 where the basis rock is syenitic greenstone, Professor Hitchcock has been able to trace 

 eleven epochs of eruption. Dykes, from the Scottish dyke, a wall or fence, are analogous 

 to veins, being generally composed of igneous matter ejected from below into fissures of 

 the strata, which are usually straight, and often form thick divisional walls. 



It is impossible to account for aH veins upon the principle of injection, as in such 

 examples as are shown in the following figures : 



Fig. 35 represents two small but very distinct granitic veins, occurring in homogeneous 

 micaceous limestone, which pursue a very tortuous course. Fig. 36 shows a granitic vein, 

 equally tortuous, and only one-eighth of an inch thick, conforming to the flexures of mica 

 schist. Fig. 37 exhibits a tortuous granitic vein, occurring in talcose slate, which does not 

 conform to the lamina? of the slate. Veins of this description are supposed by Professor 

 Sedgwick to have been produced by chemical segregation from the rock in which they 

 occur, while that was in a yielding state, just as the nodules of flint were segregated from 

 chalk, or crystals of simple minerals from the rocks in which they are now found 

 imbedded. This opinion is supported by the fact, that sometimes these veins pass by 

 insensible gradations into the intruding rock, thus showing that they are of a contempo 

 raneous origin with the rock, while both were in a fluid state. The metalliferous veins 

 are of both kinds, fissures that have been subsequently filled with metallic matter, and 

 segregations of metallic particles from the surrounding mass by elective affinity. 



The phenomena of the metallic veins constitute one of the most difficult problems of 

 geology ; but their contents are so important to human improvement and happiness, and 

 form so marked a feature of the crust of the earth, as to demand a general notice. It is 

 always the case, that the metalliferous veins, like other mineral veins, are occupied with 

 matter different from the rocks they traverse; but it rarely occurs that the metallic 

 matter, or the ore, fills the whole of the vein, but is more or less abundantly disseminated 

 through the quartz, sulphate of baryta, or gi-anite, which constitutes the matrix or 

 veinstone. The veins vary greatly in width, from a line to several hundred feet, those 

 of Cornwall being from an inch wide to upwards of thirty feet. They vary also in their 

 direction, and are of unknown depth, for scarcely ever have they been exhausted down 

 wards. The metalliferous contents likewise vary in the same veins, at different distances 

 from the surface, owing to their passage through different rocks, copper following zinc 



