TRANSACTIONS OF SECTION C. 667 



Triassic rocks of many localities and tlie Wealden and Lower Greensand of the 

 south of England, differential shrinkage may be responsible for many of the 

 smaller vertical displacements by which the beds are readjusted. Faults are some- 

 times found to dwindle and die out downward, and in certain cases these may be 

 explicable as the result of unequal contraction in masses of irregular thickness. 



On some Contorted Strata occurrinri on the Coast of Northuraherland, 

 By 3. G. GooDciiiLD, 



3. Some Facts bearing on the Origin of Eruptive lioclis. 

 By J. G. GooDCHiLD. 



The author exhibited a number of photographic slides in order to demonstrate 

 that intrusive uiasses, as a rule, replace their own volume of the rocks they invade 

 and do not cause displacement to any important extent. Hughes, and also Clough, 

 had already published evidence to the same effect. Several of the hand-specimens 

 that had been photographed were exliibited at the Meeting. In the course of 

 nearly forty years' field experience he had never met with any intrusive rock whose 

 mode of occurrence could not bs explained by the theory that the rocks in question 

 had been substituted for those whose place they had occupied. The older rocks 

 had, obviously, been gradually reinoved, and the newer ones had been, concurrently, 

 left in their place. lie thought that it was only in those cases in which the pressure 

 to be overcome had been below a certain (unknown) amount that actual displace- 

 ment occurred. This might happen where a viscid magma was being forced info 

 the loose materials in the outer parts of a volcano. These, however, are the parts 

 of a volcano which rarely survive subsequent geological changes. 



The mode of attack of the erosive magma was next illustrated by a series of 

 views representing various unfinished stages in the process. These demonstrated 

 that the intruding magma ate its way along any divisional planes in the rock 

 invaded, and by physico-chemical processes the advancing wedges enlarged and 

 extended forward, solely by peripheral solution, which ended by surrounding the 

 part attacked by the fluid magma, and thus permitting the detached iiortions to 

 float into the trunk stream. 



Further stages in the process of mastication, digestion, and assimilation were 

 shown by other slides, as well as by specimens exhibited at the Meeting. 



Three or four slides of pseudo-dykes and sills were shown. These occur within 

 fragmentary materials of volcanic origin. He considered that these clastic rocks 

 liad been softened in place and had subsequently reconsolidated in the massive 

 form without change of position. 



Finally, to account for the facts, he advanced the hypothesis that the chief 

 agent concerned in bringing about these changes was water operating under 

 pressure and at a moderately high temperature, in which were held in solution 

 the substances dissolved in sea-water. These underwent concentration by the 

 action of volcanoes, and in that state were competent not only to dissolve the 

 constittients of rocks, but to add to sedimentary or other rocks the substances in 

 which their composition is deficient as compared with that of the eruptive rocks. 

 Slow diffusion and a circulatory movement of the whole magma equalised the 

 composition of the compound. He thought that the sedimentary rocks thus 

 affected could furnish the materials for those rocks in which felspar containing 

 lime and soda predominate, while the acid series might have arisen in like 

 manner from the solution of the granitic foundation of the Earth's Crust. 



