GEOPHYSICAL LABORATORY. 149 



The manner of action of the processes is considered, not only from the geo- 

 logical side, but also from physical and chemical standpoints. A description 

 is first given of the structural relations of the gneisses as observed in the field, 

 and evidence is presented leading to the belief that at this locality the folia- 

 tion of the gneisses can not well be attributed to the squeezing-out of a partly 

 differentiated magma or to the shearing and recrystallization of a solidified 

 rock, but that its 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 a 

 laminated character. Evidence is found which indicates that the process of 

 injection was carried out in a most quiet and gradual manner, and possessed 

 many of the characteristics of a substitution of the original material by the 

 magmatic solution rather than the features of a violent intrusion. The 

 observed relations are very similar to those which certain French geologists 

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

 tion is believed to have been essentially the same. 



Certain features which were observed in the gneisses imply properties of 

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

 degree of viscosity implied by the presence of thinly tabular sheets of inclu- 

 sions within the granite, standing nearly upright 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 physical and chemical 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 question of the 

 possibility of a viscous magma penetrating the pores of the wall-rock is con- 

 sidered and the problem of a possible differentiation of a magma when injected 

 between the layers of a rock in a multitude of adjacent streams is taken up. 

 Certain inferences are drawn regarding the operation of such processes and the 

 conclusion is reached that under such conditions of injection as prevailed at 

 this locality, 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 concentrated body following would carry farther toward completion. 



(27) The analcite basalts of Sardinia. Henry S.Washington. J. Geol., 22, 742-753 (1914). 



At Monte Ferru and elsewhere in Sardinia lavas occur which show in thin 

 section small round isotropic areas, resembling sections of leucite, so that the 

 rocks have been commonly held to be leucite basalts. A careful study and 

 three chemical analyses of these rocks, collected during the author's trip to 

 Sardinia in 1905 for the Carnegie Institution of Washington, show that the 

 supposed leucite is in reality the hydrous soda mineral analcite, and that it is of 

 primary origin. Comparison ^vith similar rocks from other regions indicates 

 that some so-called leucitic rocks are in fact analcite-bearing, and that rocks 

 containing primary analcite in well-developed crystals are much more abun- 

 dant than has been supposed. Analyses are also given of the augite and 

 olivine which form nodules in one of the lavas, the optical study of which is to 

 be taken up later. 



(28) I Basalti Analcitici della Sardegna. Henry S. Washington. Boll. Soc. Geol. Ital., 33, 



147-167 (1914). 



An Italian translation of "The analcite basalts of Sardinia" (J. Geol., 22, 

 742-753: (1914). Reviewed under No. 27 above. An appendix is added which 

 gives a brief outline of the quantitative classification of igneous rocks. 



