FENNER, THE WATCHUNG BASALT 113 



The constant removal of material in solution undoubtedly enlarged the 

 channels of circulation and, in places, produced cavities of considerable 

 size. The most perfect crystals are naturally found in vugs of this kind. 

 It does not appear, however, that such openings were at all necessary for 

 recrystallization, and in most cases they were not present. The leaching 

 appears to have been somewhat selective, for iron and magnesia are 

 greatly reduced in the later stages. 



II. Petrography of the Secondary Minerals 



METHODS OF STUDY OF MUTUAL RELATIONS OF MINERALS 



All the material for petrographic work has been obtained from the 

 region lying between Paterson and Montclair Heights. In many places 

 throughout this district, natural exposures of surface rock show evidences 

 of the features which have been ascribed to underlying lake beds; but 

 weathering has produced its usual effects of solution and decomposition, 

 and such exposures do not afford desirable material for investigation. In 

 a number of places, however, quarries have been opened for the purpose 

 of obtaining road material or railroad ballast, and the fresh rock thus 

 brought to view offers all the desired opportunities for study. A quarry 

 at West Paterson and two at Great Notch have furnished the bulk of 

 the material, but a considerable amount has been obtained from the 

 waste-dump of a tunnel driven through the mountain near Great Notch a 

 number of years ago for a water-supply system. 



Before beginning the microscopic study of the relations which the 

 various minerals bore to each other, it had been observed from macro- 

 scopic specimens that there was probably a definite order of deposition, 

 and the writer presented a tentative determination of the sequence in a 

 previous paper,^'^ although stating that there were often great difficulties 

 in coming to a decision. As the petrographic work progressed, it was 

 found necessary to revise previous ideas along several lines. Certain 

 criteria which had been followed in accordance with general conceptions 

 were found in this instance to be unreliable guides. The manner in 

 which bands of granular calcite frequently appear between unaltered 

 basalt and other secondary minerals may be cited as an example. The 

 inference would be that the calcite was of earlier formation and was the 

 first to be deposited on the basalt, but in numerous instances, it has been 

 found to be later, and the latter relation is believed to be the general one. 

 Similar effects appear with other minerals. Not infrequently, partial 



" Jour. Geol., vol. 16, pp. 297-327, 1908. 



