SCROPE — ON INTERNAL STRUCTURE OF GNEISSIC ROCKS. 365 



Mr. Sorby's experiments, in "which he produced slaty lamination m 

 a mixture of pipe-clay and glue containing flattened particles of mica 

 or oxyde of iron, by subjecting it to a squeeze, exhibit the same 

 result. 



"We may now proceed to apply these considerations to the case of the 

 crystalline rocks of igneous origin. It has been calculated that granite 

 must lose one-tenth of its bulk in the process of crystallization or con- 

 solidation from a state of fluidity. Then, on the other hand, it must 

 augment in volume by one- tenth on passing from a solid to a fluid state. 

 Whether this be the precise proportion or not, it is quite certain that 

 the changes of temperature to which granite and all other igneous rocks 

 have been exposed, must have been accompanied by a corresponding 

 change of volume, even while its component minerals still retained 

 their crystalline state. Eut every such dilatation or compression must 

 occasion a considerable amount of motion and consequent friction of the 

 component crystals inter se. If, as there is great reason to believe, 

 water has been always present throughout the crystalline igneous rocks, 

 holding much silex in solution, perhaps in a gelatinous state, this will 

 have acted as a lubricator to the more solid and still crystalline 

 minerals, and enabled them to move among themselves in a consider- 

 able degree without being completely broken up, as might be otherwise 

 expected, from the amount of friction to which every change of volume 

 must have subjected them. Under such circumstances, we can conceive 

 a mass of crystalline rock to undergo considerable dilatation through an 

 increase of temperature without much change in the form or position of 

 its component crystals, on the supposition that the pressures to which 

 it is exposed be nearly equal on all sides, and consequently its expan- 

 sion nearly equal in all directions ; although, in this case, many frac- 

 tures, cracks, and crevices occasioning a brecciated or veined structure 

 may be produced. But, should such an expanding mass force its way 

 upwards through a rent broken across a vast weight of overlying strata, 

 tilting these latter and shouldering them off on either side, the upper 

 and lateral portions of the elevated mass must be subjected to such an 

 oblique and unequal strain or squeeze between the downward pressure, 

 and perhaps lateral movements, of the tilted strata and the upward 

 thrust of the axial crystalline mass, as must tend to break up more or 

 less the component solid particles or crystals of these lateral portions, 

 and arrange their fragments with their longest axes in the direction of 



