114 ANXALS NEW YORK ACADEMY OF SCIENCES 



replacement causes an earlier mineral to appear to rest upon a later, and 

 often those minerals which have an acicular habit seem to have had the 

 ability to penetrate without difficulty through earlier-formed minerals of 

 a more massive nature. For these reasons, it is believed that macroscopic 

 observation alone is unreliable. On the other hand, microscopic observa- 

 tions have certain limitations, and it has been found advantageous to 

 employ both methods as supplementary to each other. 



In some respects, the zeolites are difficult subjects for petrographic 

 study in random rock sections, when it is desired to differentiate the 

 various members of the group, because of the rather negative character 

 of the optical properties and the resemblances among difEerent species. 

 By careful study of known sections, however, it was found that difEerences 

 of habit could be perceived which were often sufficiently distinctive for 

 rapid identification, when checked in doubtful cases by further tests. 



Throughout the various portions of the district covered, the relations 

 of basalt and secondary minerals appear to be almost identical. There 

 are differences in the same quarry which are as great as in parts of the 

 field several miles apart. Such differences embrace the greater or less 

 prevalence of the bowlder-structure in places and the variations in thie 

 predominant minerals. The latter is probably an effect of the somewhat 

 variable amount of sublimation products condensed in the crevices and of 

 the facility with which solutions percolated through the channels. 



LIST OF MINERALS 



In the hand specimens the following minerals are observed to occur in 

 quantity: quartz, datolite, prehnite, pectolite, analcite, chabazite, heu- 

 landite, stilbite, natrolite, laumontite, apophyllite, thaumasite and cal- 

 cite. Gypsum is found, but is much more rare. Pyrite and chalcopyrite 

 are frequent, but almost always in small grains. Galena and sphalerite 

 are known to occur, but they have not been met by the writer. Hematite 

 is often noticeable as a finely divided purplish dust disseminated through 

 crystals of silicates and calcite. Tabular hematite crystals of larger size 

 also occur. Gmelinite has been reported, but in a number of instances 

 which have come under the writer's observation the specimens appeared 

 to be more probably chabazite. The two, however, doubtless form a con- 

 tinuous series analogous to the plagioclases, and considerable variability 

 in composition is to be expected. The relation of chabazite and of gmel- 

 iiiite to other zeolites probably presents no essential difference, and no 

 attempt was made to distinguish the two in microscopic work. Scolecite 

 has also been reported, but the writer has not met with satisfactory ex- 



