110 H. A. Daly — Mechanics of Igneous Intrusion. 



possible to tread on sure theoretical ground in dealing with the 

 origin of olivine nodules in basalt, much less in accounting for 

 the more important basic segi'egations of granites, syenites, 

 etc. It is noteworthy that, besides olivine, augite, hornblende 

 and oligoclase appear to have unusually high coefficients of 

 cubical expansion, and that these minerals are among the com- 

 moner constituents of segregations in granites and syenites. 

 The highly important constituent, biotite, has not been, per- 

 haps cannot be, directly investigated in this regard. In the 

 absence of experimental data, the further discussion of basic 

 nodules in the present connection cannot afford very fruitful 

 conclusions. All that seems certain is that the existence of 

 basic segregations does not disprove a high degree of fluidity 

 for plutonic magmas. 



Shattering at plutonic contacts. 



Among the commonest phenomena associated with the con- 

 tact-zones of plutonic, igneous rock-bodies (bosses, stocks and 

 batholiths) is that of extensive shattering and disruption of the 

 invaded formations along the contacts. A host of memoirs on 

 exotic granite, syenite, diorite, gabbro and other deep-seated 

 rock-masses contain references to this particular phenomenon. 

 It consists, in its ideal development, of the appearance of two 

 concentric zones of mixed rock occurring between the homo- 

 genous main body of igneous material and the encircling 

 country-rock unaffected by any serious mechanical disturbance 

 due to the intrusion. Both zones lie parallel to the average 

 line of contact between the intrusive and the country- rock. 



The Zone of Apophyses. — The belt more remote from the 

 intrusive body is generally much the broader of the two and 

 consists of country-rock intersected by more or less numerous 

 apophyses from the main igneous mass. These dikes and 

 sheets are often seen to radiate outward in directions roughly 

 normal to the average line of contact ; but others running at all 

 angles to the contact are usually associated. The whole group 

 of apophyses has often thus a reticulate ground-plan and a 

 reticulate vertical section. The portions of the invaded for- 

 mation bounded by the apophyses are not essentially disturbed 

 from the relative positions they occupied before the intrusion 

 took place. This belt may be called " the zone of apophyses.'^ 

 According to the prevailing views among geologists, the zone 

 owes its origin to a mechanical process of relative simplicitj^, 

 namely, the injection of the molten magma forced by great 

 pressure into all accessible planes of weakness within the invaded 

 formation. The force may be hydrostatic, due to the weight 

 of the chamber-roof or of part of that roof; or, among other 



