288 Dr. J. E. Reynolds on certain Mineral Silicates. 



unless some stronger affinity than that of the SiO 6 comes into 

 play ; yet its tendency is to pass into leucite, which mineral 

 would result from the addition of SiO 6 to the above group. 

 Though it cannot undergo the change by loss of base, it can, and 

 no doubt does, by increment of acidulous radical; it will seize any 

 silica which the necessity of the compound molecule enables it 

 to displace from feeble combinations. The result of this is that 

 the SiO 6 is, or its elements are, condensed within the molecule 

 of the silicate, the tendency to accumulation of the polyatomic 

 radical is partially satisfied, and leucite is the result. Yet this 

 is supposed to take place in the magma which on cooling gives 

 the volcanic rocks; and Durocher tells us, and numerous ana- 

 lyses prove amply, that these rocks are essentially basic, and 

 comparatively poor in silica. It would appear, then, that the 

 tendency of the acid radical of the ideal silicate to reach the second 

 stage of condensation is sufficient to enable it to overcome the 

 opposing influences around it in order that leucite may be pro- 

 duced ; but the cumulative power is not enough to enable it to 

 pass into orthoclase by the appropriation and condensation of 

 the silica of other compounds. When we come to the granitic 

 rocks, however, no such paucity of silica is observed; on the 

 contrary, we have this body largely in excess, and the necessary 

 consequence is, that in these acid rocks we meet with orthoclase, 

 a mineral in which the acid radical appears to have reached its 

 greatest proportion, and the silica to have completely satisfied its 

 tendency to accumulation within the molecule. 



In concluding this section, I may remark that the third stage 

 of condensation of SiO 6 is the highest which appears to have 

 been reached in any mineral. Dana* regards the presence of 

 potassium and sodium in a felspar as the condition which deter- 

 mines the greater or less proportion of silica in such a compound. 

 There appears to be much truth in this ; but I think the 

 genesis of a mineral, as instanced above in examining the rela- 

 tions of leucite and orthoclase, should also receive full attention. 

 When this important point is weighed in connexion with the 

 explanation above offered of the causes which have probably 

 operated in the production of one or other mineral, we see still 

 more clearly than before the profound isotypism of the felspar 

 family; and not only this, but we have distinctly brought out 

 the peculiar rate of condensation of silicon within the molecule 

 of some silicates which it has been the object of the present 

 paper to call attention to as affording a simple means of classi- 

 fying the salts of normal and polysilicic acids. 



* American Journal of Science, vol. xliv. p. 401. 



