3g ANNALS NEW YORK ACADEMY OP SCIENCES 



Essential volume change"' = -\- SO. 31 per cent. 



It will be noted that incipient development of both sepiolite and dewey- 

 lite from mineral decay has been attended by separation of a certain, 

 amount of magnesium hydrate. This was not understood, except by Eoth 

 and Teal, or included in the formulas previously given; it may serve as a 

 test of the truth of the reaction here set forth. 



Later, with subjection of deweylite to the thermal conditions in a 

 lower metamorphic bolt, the complementary process of alteration has 

 taken place: 



3(II,2Mg,Hi3O,6 + naq.) + A = 4H,Mg,Si,A + SiO, + 10H,O + naq. 



i;<)ll<)i(l ili'ueylite Meal Antigoiite (ii4.« Hyalite 



percent.) or qiiaitz. 



Essential volume ehange^^ = — 32.79 per cent. 



Reference has already been made to commonly accepted views concern- 

 ing dynamic effects upon bodies of serpentine by the changes of volume 

 in progress during passage of minerals into talc and antigorite. It is now 

 apparent tliat admission into the equations of the hydrated colloids of 

 sepiolite, deweylite, etc., actually found in nature, would involve an early 

 liypothetical expansion far greater than hitherto estimated. On the 

 other hand, the later physical changes which have preceded the birth of 

 talc and of antigorite have generally culminated in notable contraction 

 of the rock mass. We have to do here, however, with more than chemical 

 reactions. The attendant physical processes of solution, leaching, trans- 

 port and migration of soluble constituents, and their later alteration in 

 a deeper thermal zone, have resulted in a complex Assuring, and often in 

 an amount of contraction which has decidedly offset the expansion from 

 early chemical changes. The observed evidences of internal disruption 

 and movement in bodies of serpentine may be therefore everywhere ex- 

 plained, I judge, by successive thi'oes of expansion and contraction — r. g., 

 at 8taten Island and Xew Rocliclle, New York ; Montville and Hoboken, 

 New Jersey, and Thetford, Canada — and also by local strains and faults 

 produced by orogenic disturbances. 



P'or precise definition of processes above considered, I think we need 

 differentiation of the following terms: 



Decay of rocks, to express the result of operations within the belt of 

 weathering, disintegration, oxidation and extreme hydration. Among 

 the more important products are the colloid magnesium hydrosilicates of 

 the first type (colloid deweylite, sepiolite), magnesium oxide, hydrate 

 and giobertite, besides various forms of ferrous and ferric hydrates, hy- 

 drocarbon ates, etc. 



'" Without repard to n i\q. 



