G. Poulett 8crope — On '^ Blochy" Rock Surfaces. 293 



cisely the same rate of refrigeration, abstracting, of course, local and 

 accidental variations of atmospheric temperature. 



I would suggest, then, that whenever the surface of an igneous 

 rock, such as those described in Mr. Ward's paper, is seen to be 

 formed of tumbled blocks, and split by gaping fissures, it is pre- 

 sumably the upper portion of a protruded mass which reached its 

 present position in a state of igneo-aqueous liquefaction, and rapidly 

 parting with its contained steam and heat on exposure to the 

 atmosphere, was rent and broken up by sudden shrinkage in the 

 manner described above as characteristic of many fields of coarse- 

 grained and highly crystalline lavas. As the process of consolidation 

 and cooling extended downwards, the outwardly gaping fissures will 

 have been continued in closer vertical joints ; or, where the separa- 

 tion of the sides was considerable, these would be filled by the 

 finest grained material, or perhaps, liquefied silica, squeezed out 

 from the contiguous crystalline mass, giving rise to the veins of 

 finer crystalline matters, or of pure quartz, with which these rocks 

 are so often penetrated. 



Much that seems obscure in the position, structure, and texture 

 of the crystalline igneous rocks of all ages, as also in the influence 

 they have exercised on the stratified rocks with which they came in 

 contact, would come out far clearer if it were borne in mind that these 

 rocks assumed their present positions while in the state of a more 

 or less pasty magma, highly elastic, i.e. having a tendency to expan- 

 sion, by reason of the quantity of super-heated water or steam dissemi- 

 nated through its substance among the component imperfectly formed 

 crystals or granules ; — a view which I advanced nearly fifty years 

 back, and which has since been accepted by many eminent geologists. 

 For example, the more or less horizontal pressure that has evi- 

 dently been exerted in frequent instances by plutonic masses upon 

 the strata through which they have burst up, shouldering aside and 

 crumpling them into anticlinal and synclinal curves, or causing 

 complicated faults in them when too rigid to bend — which forms the 

 subject treated of by the Eev. 0. Fisher in the elaborate article which 

 follows that of Mr. Clifton Ward — is thus readily explained ; since 

 any elastic mass placed between the two opposite forces of the up- 

 thrust from below, and its own weight from above, must tend 

 to. expand horizontally with enormous power. There would seem, 

 then, to be no need to resort, for an explanation of these phenomena, 

 to the hypothesis of a shrinking nucleus to the globe. The rise 

 from beneath the fractured crust of masses of granite or other 

 plutonic crystalline matter in a state of igneo-aqueous plasticity, and 

 necessarily exerting an enormous horizontal, or rather diagonal, 

 pressure against the sides of the fissure through which the matter is 

 rising, or in any direction that ofi"ers the least resistance, — together 

 with the tendency of the upraised strata, under the influence of 

 gravitation, to slip away from the axis or centre of elevation, — will, I 

 think, account for all the evidences of lateral thrust observable in 

 the faults and crumpling of the strata.^ 



1 See Volcanos, p. 287, ed:. 1872. 



