736 DYNAMICAL GEOLOGY. 



The effect, in a melted spheroid, of cooling more rapidly at the sur- 

 face than within, is illustrated in glass in a Prince Rupert's drop. 

 The pressure of particle against particle over the whole exterior is so 

 great, from the interior contraction, that the removal of a portion of 

 the surface-layer by a slight scratch of a file destroys the equilibrium, 

 and causes it to break instantly, and almost explosively, to fragments. 

 Another familiar example of contraction beneath an exterior coat is 

 seen in a drying apple. The exterior, in this case flexible, gradually 

 becomes wrinkled, from the diminution of size within ; and the wrink- 

 ling covers the whole surface alike, unless some part be protected by 

 resin or otherwise, — in which case the largest wrinkles would be those 

 about the border of the protected portion. 



3. Constitution of the Earth's liquid Exterior, and of the cooled 

 Crust made from it. — The nature of the first-formed crust, or of the 

 liquid material of which it was made by cooling, may be inferred (1) 

 from the materials which have come up through tappings of the inte- 

 rior, that is, igneous rocks ; and (2) from the nature of the earliest 

 rocks of the supercrust, the Archgean. 



Igneous rocks show, by the fact that four-fifths consist of the lime- 

 feldspar, labradorite, and the iron-bearing silicates, augite, hornblende 

 and chrysolite, together with magnetite (the iron oxyd Fe 3 4 ), the con- 

 stituents of doleryte, that four-fifths of the true crust are probably 

 dolerytic, or basic, and iron-bearing ; while the remainder of the igne- 

 ous rocks — the feldspathic kinds, related to trachytes — evince that 

 it has its regions of potash and soda feldspars (orthoclase and oligo- 

 clase), nearly free from iron-bearing minerals. The former kinds have 

 mostly a specific gravity of 2*9 to 3*2, and the latter of 2*5 to 2-75, 

 and hence the mean specific gravity of the true crust is probably 

 about 2*9. 



Among Archaean rocks, hypersthenyte has nearly the constitution 

 of doleryte ; other kinds, containing chrysolite, are closely related to 

 the dolerytic rock, peridotyte ; diabase has the composition of a chlo- 

 ritic doleryte ; so that these first deposits over the crust, made from its 

 detritus, suggest the same conclusion as the igneous rocks. Moreover, 

 iron-bearing Archaean rocks greatly exceed in amount all others ; and 

 the iron-ore beds of the Archaean are vastly larger than any of later 

 time, — some exceeding a hundred feet in thickness. Hence, unlike 

 human history, the earth's iron-age was its earliest. 



The distribution of the dolerytic, or basic, and the trachytic, or acidic, portions of the 

 crust was probably determined by the general currents over the sphere when it was 

 liquid, and also by local movements dependent on special centres of igneous activity. 

 This cause would naturally have led to a more or less perfect separation of the less fusi- 

 ble and lighter feldspathic portion from the rest. Daubr^e has inferred the occurrence 

 of a large proportion of chrysolite in the true crust, from its prevalence in meteorites. 



