Ch. XVI.] OF THE PREVAILING THEORY. 383 



be arranged, side by side, in a regular and conformable order, 

 as at the junction of large masses of these rocks, or, in the 

 case of their association, in lesser or alternating beds ; nor, 

 that the individual concretions should frequently contain both 

 rocks in different proportions, and yet exhibit a distinct line 

 of junction, as denoted by the colour or the texture of each 

 mineral compound : for since the various parts of the whole 

 mass graduate into each other, it can be easily imagined that 

 the elective chemical attractions which regulated the com- 

 binations of the particles of the cooling fluid mass, might be 

 independent of the mechanical attractions of cohesion, by 

 which the concretions were formed during its consolidation. 

 Again, as regards the internal structure of these rocks, we 

 have seen that it may be either compact or schistose, in all 

 without exception ; though the former prevails in some places 

 and the latter in others : these kinds of structure can, there- 

 fore, only be considered as specific, not as generic distinctions : 

 and much less as indications of different modes of formation, 

 affording a proper basis for the division of rocks into the 

 stratified and unstratified classes. We have, elsewhere, at- 

 tempted to illustrate this subject by referring to the structure 

 of an ingot of bismuth, which has been allowed to cool very 

 slowly from a state of fusion ; it exhibits clusters of crystals, 

 both rectangular prisms and octahedrons, surrounded by 

 foliated laminae; affording a striking representation of the 

 formation of the massive and schistose structures in one and 

 the same mass, and that too in a simple elementary substance. 

 When we descend to the composition of the granitic and 

 schistose rocks it will be found to furnish additional evidence 

 of their contemporiety. Thus, each group of these rocks 

 shows that all its members, however dissimilar in their ap- 

 pearance, repeatedly pass into each other by the most insensible 

 gradations ; and not only so, but these groups have the same 

 relations, wherever they meet together. This character is so 

 strongly marked, that even stratified limestone, where it joins 

 granite, as at Glen Tilt, partially loses its fissile structure, and 



