702 CLARENCE N. FENNER 



of a partly differentiated magma or to the shearing and recrys- 

 taUization of a soHdified rock, but that their origin must be looked 

 for in a process involving the injection of a thinly fluid granitic 

 magma between the layers of an original rock of laminated struc- 

 ture. Structures which still survive in the larger bodies of granite 

 indicate that the process of injection was carried out in a most 

 quiet and gradual manner, and possessed many of the characteris- 

 tics of a substitution of the original material by the magmatic solu- 

 tion rather than the features of a violent intrusion. The observed 

 relations are very similar to those which French geologists have 

 described under the name of lit-par-lit injection, and the mode of 

 operation is believed to have been essentially the same. 



Certain features observed in the gneisses imply properties of 

 the magma which at first sight do not appear mutually consistent. 

 Thus the degree of viscosity indicated by the presence of thinly 

 tabular sheets of inclusions within the granite, standing nearly up- 

 right and unsupported except by the magma on either side, does 

 not harmonize with the facility with which magmatic material 

 has been transfused into the original rock. In trying to reconcile 

 these features inquiry has been directed toward a consideration 

 of certain of the properties of magmatic solutions. The question 

 of the critical temperatures of volatile substances is discussed in its 

 bearing upon their condition within the magma. Further, the 

 problem of a possible differentiation of a magma when injected into 

 a wall-rock in a multitude of adjacent streams is taken up, as 

 related to the views expressed by the advocates of lit-par-lit injec- 

 tion. Several methods by which this might be accomplished 

 appear possible, and it is suggested that the escape of gases from 

 the magma into the wall-rock would have a tendency in this direc- 

 tion and that at the same time the deposition of magmatic minerals 

 by contact of tongues of magma with the cooler walls would effect 

 results of a similar nature. Thus the advance of the main body of 

 magma would be preceded by that of a more dilute portion, which 

 would be able to impregnate the wall-rock with facility and initiate 

 processes of transformation and solution which the more concen- 

 trated body following would carry farther toward completion. 



December, 19 13 



