370 PEOCEEDINGS OF THE GEOLOGICAL SOCIETY. [JuHG 19, 



this has not been owing to an insufficient quantity of the acid ; for 

 in the " diorites," in which the silica is in the same atomic propor- 

 tion as in the traps, the combination of the bases with the silica is 

 complete. In the smaller dykes, the extraneous elements have 

 formed a larger proportion of the erupted rock than in the larger 

 masses; and in these last the original bisilicate has been most 

 alloyed in that part of the molten rock which must have come 

 first to the surface. As other portions rose in a plastic state 

 and formed successive layers, forcing the earlier portions out, by 

 distention, from the centre, the central portions have become less 

 and less affected by the assimilation of foreign matters ; and so they 

 approximate nearer to the composition of the bisilicated mass from 

 which I have supposed them to have been previously derived. 



6. In all the rocks of the Malvern chain, known or supposed to 

 be products of eruptive action, the atomic proportion of the silica to 

 the bases varies according to a common law. In the traps and 

 augitic lavas, the ratio of the oxygen of the silica to that of aU the 

 bases chemically combined with it is never greater than 2 : 1, nor 

 less than 3:2. In those which I have described as trachytic rocks, 

 this ratio is never more than 3:1, nor less than 5:2. In the 

 granitic veins it is not more than 4:1, nor less than 7:2; and 

 in some quartzo-felspathic and petrosiliceous rocks it is not greater 

 than 5 : 1, nor less than 9 : 2. The eruptive rocks may thus be pri- 

 marily referred to four sources, in which silica was combined with 

 bases according to a regular arithmetical progression, which may be 

 expressed by the numbers 2, 3, 4, 5, other elementary substances, 

 acid and basic, being united according to their several elective 

 affinities. But when such of these as were decomposable by heat 

 were exposed to a high temperature in foci from which they were 

 eventually ejected, many bases were necessarily separated by the 

 evolution of their acids in a gaseous form. And as silicic acid will 

 combine with bases in a great variety of proportions, and its power 

 of forming such combinations was increased by the same circum- 

 stances under which that of other acids was destroyed, the silica 

 appropriated to itself a larger proportion of base than that with 

 which it was at first united. But the extent of this alteration was 

 in all cases limited ; and its maximum amount may be expressed 

 arithmetically by the proportions 3, 5, 7, 9 to 2. And enough of 

 the original silicate still remains, especially in the central parts of 

 the largest masses, to enable us to refer them to one or other of the 

 four well-defined classes into which they are naturally divided, and 

 to show that they were formed by the combination of the silica with 

 bases according to the usual law of multiple proportions, its quan- 

 tities in different compounds bearing a very simple relation to each 

 other — a combination which could only have taken place with such 

 regularity where the various elements were in such a state of atomic 

 division, and possessed such perfect fj-eedom of motion, and such 

 complete exemption from all disturbing forces, that the action of 

 chemical affinity was entirely unrestrained.. 



