400 Dr. Mac Cull och on certain colours in Killas. 



coloured fluids of considerable density or tenacity: should the layers 

 of the several fluids have been straight, the curved and wavy ap- 

 pearance is given by producing short and partial disturbances in 

 different parts of the compound. There can be very little question 

 that this rock must have been coloured by a similar operation while 

 in a semifluid state, for on no other hypothesis can the peculiar 

 distribution of the two coloured substances through the whole mass 

 be explained. The continuity of the lines of colour precludes all 

 possibility of a succession of deposited layers, otherwise than in 

 those very lines, and affords at the same time a proof, if any were 

 wanting, that the fissile property of this killas has not been the 

 result of stratification. The whole must in fact be considered as 

 formed either of one deposit, of a semifluid red mud, coloured 

 afterwards by a mixture of blue mud, or of successive layers of 

 red and blue mud. In this state the application of external dis- 

 turbing force has produced the peculiar contortion here exhibited. 

 It is evident that the theory of softening used to explain the con- 

 tortion of rocks, is in this case insufficient : a species of fluidity is 

 requisite, otherwise the elongation and narrowing of the blue lines, 

 could not have taken place. 



Having established the necessity of consolidation from a fluid 

 state, it remains to ascertain by what powers both the fluidity and 

 the consolidation were effected. There is no difficulty in supposing 

 that the requisite state could exist in a mere mixture of clay and 

 water at the ordinary temperature : but when we consider the large 

 proportion of water requisite to maintain that state in a given 

 quantity of clay, it is difficult to conceive how the disposition of its 

 parts could have been preserved during the great contraction which 

 it must have undergone in the act of consolidation. 



