224 University of California Publications in Geology [Vol. 8 



Now, for the purpose of demonstrating the possibility which Lind- 

 gren denies or doubts, we may assume that an intrusive mass has 

 invaded a syncline of sedimentary strata, including limestone, within 

 the zone of fracture for strong rocks. As a mechanical consequence 

 of a sudden intrusion the structure of the encasing rocks will be dis- 

 turbed or modified. If it be a sill it will lift the superincumbent strata 

 and there will be stresses and ruptures inaugurated on its margin. 

 Radial ruptures may be occupied by dykes ; tangential ruptures may 

 escape such occupation. The lifted strata may be broken into blocks 

 faulted against one another. 8 If it be a plano-convex laccolith, besides 

 similar stresses and ruptures there will be distension of the overarch- 

 ing strata in a central position. If it be a steep-sided laccolith, the 

 tendency to peripheral tangential rupturing will be pronounced and 

 effective. If the intrusive mass, on the other hand, make its way 

 slowly into the syncline by a process of stoping or resorption or both, 

 there may be a certain amount of shattering and faulting of the sedi- 

 mentary rocks on its periphery. 9 These results of the intrusive act, 

 in so far as they fracture the encasing rocks, provide channels for the 

 subsequent circulation of vapors and waters, but their efficiency as 

 conduits may for a time, particularly at the immediate periphery of the 

 intrusion, be greatly reduced by the expansion of the encasing rocks 

 due to rising temperature. 



Probably a more important development of conduits for the cir- 

 culation of vapors and water is that which follows the solidification 

 of the intrusive mass by reason of the shrinkage due first to crystal- 

 lization and then to subsequent loss of heat. Such shrinkage would 

 give rise to stresses that could be relieved only by distensional frac- 

 tures, many of which would doubtless be partially sealed by normal 

 faulting and the gouges thereby produced. That such shrinkage cracks 

 are important features of cooling igneous masses and may become the 

 conduits of water circulation and ore deposition is being freely 

 recognized in recent years, notably by Pirsson, 10 Weed, 11 Barrell, 12 

 Miller, 13 and Tyrrell. 14 It is to be noted that such shrinkage cracks 

 are not confined to the encasing rocks of the intruded mass, but may 



sCf. Eansome, U. S. G. S. Professional Paper no. 12, pp. 97-106. 

 » Cf. Barrell, U. S. G. S. Professional Paper no. 57, pp. 99-104. 



10 Am. Journ. Sci., 3d ser., vol. 50, 1895, p. 116. 



11 Trans. A. I. M. E., vol. 33, 1903, p. 745. 



1 2 U. S. G. S. Professional Paper no. 57, p. 104. 



is Quoted by Tyrrell. Original paper not available. 



14 Canadian Mining Journal, August 1, 1907, pp. 301-305. 



