University of California. 



[Vol. i. 



of rock, and that neither direct observations in the field nor infer- 

 ences from the general distribution of the rocks suggest that it 

 occupies a peripheral relation to the mass of the laccolite. In 

 current discussions on magma differentiation it is usually assumed 

 that the more basic portions of the products of crystallization in a 

 magma basin are to be found on the periphery of the mass. This 

 is undoubtedly true in man}- well-known cases. That it should be 

 laid down as a general law is, however, questionable. The familiar 

 phenomena of basic secretions in various rocks seem to have no 

 genetic connection with the periphery of the masses in which they 

 occur. Under certain conditions it is entirely conceivable, and even 

 probable, that these basic secretions may acquire dimensions rela- 

 tively great and be properly regarded as products of magma 

 differentiation, as now understood, and yet have no connection with 

 the periphery of the mass of the magma. Of such a character 

 appear to be the areas of nepheline-pyroxene-malignite in the 

 Poohbah Lake laccolite. The relative distribution of the other 

 two types of malignite is not well known beyond -the N fact that the 

 one prevails on the northern shores of the lake and the other on 

 the southern. It is to be remarked, however, that, taking our 

 orientation from the stratification of the Coutchiching rocks, the 

 garnet-pyroxene-malignite is at the lower part of the mass and 

 the amphibole-malignite at the upper part. From the further fact, 

 however, that the laccolite mass has, as a solid rock, partaken of 

 none of the disturbance, which has affected the Coutchiching series, 

 it is probable that the latter was upturned prior to the invasion of 

 the laccolite, so that our conceptions of the upper and the lower 

 parts of the laccolite must be checked by this consideration. 



The most striking fact which a review of the petrographical 

 descriptions discloses is that intense!}' contrasted mineralogical, 

 structural and textural variations are the concomitants of very 

 moderate changes in chemical composition. This is well illustrated 

 by a comparison of the analyses of the garnet-pyroxene-malignite 

 and the amphibole-malignite. 



