t6 4 OUT 1. 1 M-s OF FIELD-GEOLOGY 



further than an inch or two into the rock ; while along many 

 dykes the shales show scarcely any perceptible harden- 

 ing. It is where they come in contact with ( arhonaceous 

 shales, or still more with seams of coal, that dykes .m<l 

 intrusive sheets produce their most marked metamorphism. 

 The coal has sometimes been entirely consumed, and a 

 layer of igneous rock has taken its place. At other times 

 a thin sheet of molten lava has been injected along the 

 top, bottom, or centre of the coal-seam, converting it into 

 a kind of anthracite or into a mere cinder. Examples 

 may be found where the coal has been fused into a 

 cellular mass, and has subsequently had its vesicles filled 

 up with infiltrated carbonate of lime. In Ayrshire 

 numerous beautiful sections have been laid bare, where 

 the coal has been rendered prismatic, the hexagonal or 

 polygonal prisms, like so many bundles of pencils, 

 diverging from the surface of the intruded igneous rock. 

 At the same time, it is where they have inflicted such 

 injury upon coals and carbonaceous shales that the igneous 

 masses have themselves experienced most alteration. 

 The most solid black crystalline basalt, where it runs 

 through one of these strata, is changed into a pale dull 

 yellow or white clay, so deceptively like some of the fire- 

 clays of the coal-fields that it will hardly be admitted by 

 most observers to be anything else until they trace out 

 its relations. But they may find it passing insensibly 

 into the ordinary condition of basalt or diabase, as it 

 recedes from the carbonaceous bed the combustion of 

 which has reduced its oxides. A thin section of one of 

 these " white traps," placed under the microscope, still 

 shows traces of the original crystalline structure of 



