14 MR W. F. P. M'LINTOCK ON THE ZEOLITES AND ASSOCIATED MINERALS 



the taction of magmatic solutions. C. N. Fenner* has given fall descriptions of the 

 mode of occurrence of zeolites and other minerals in the Watchung basalt, New 

 Jersey. He shows how the minerals were deposited in a definite order from aqueous 

 solutions and how the earlier anhydrous and slightly hydrous ones are succeeded and 

 replaced by the later-formed, more hydrous compounds. He considers that the 

 zeolites and their associates were deposited during the cooling of the basalt, but, from 

 various facts connected with the occurrence, concludes that the water which caused 

 the change was derived largely from the underlying sediments. f 



II. Petrography of the Vesicles. 



Confining ourselves for the present to the well-defined scolecite-bearing zone on 

 Maol nan Damh, we find that, under the microscope, the rock proves to be a typical 

 olivine basalt. Fresh olivine is absent, but chloritic pseudomorphs after it can be 

 detected. The augite occurs in somewhat large sub-ophitic plates and is of the 

 purple colour so common amongst the Tertiary basalts ; it is frequently altered to 

 chlorite, but much of it is quite fresh. The felspar, which occurs in small laths 

 piercing the augite, contains veins and inclusions of chlorite and has been albitised. 

 Owing to the abundance of inclusions it is often difficult to apply the Becke test to 

 the laths ; but whenever a determination could be made the refractive index was 

 found to be below that of balsam, and the mineral appears in most cases to be albite 

 or, at least, a very acid plagioclase. In the coarser portions of the basalt the 

 albitisation is very pronounced, and sometimes — presumably when the original felspar 

 was highly basic — crystals are now represented merely by a chlorite pseudomorph 

 with or without an albite rim. This point has been observed in some of the rocks 

 from Devonshire.! The peculiarity of albitisation is of general occurrence in the 

 rocks around the plutonic centre in Mull, and, as epidote is also characteristically 

 present, an interdependence of the two phenomena is suggested. 



In addition to the above-mentioned minerals, black oxide of iron is also present 

 in fair quantity in the basalt. 



Sections cut through the junction of the rock with the amygdales show interesting 

 features which not only reveal the order of events during the deposition of the 

 vesicle-minerals, but also suggest that they were deposited during the cooling of 

 the lava itself. The identification of the minerals is a comparatively simple matter, 

 and, owing to the abundant development of the various species on the specimens 

 selected, the determinations can usually be checked by an examination of the 

 various powders in oils of suitable refractive index. In general the zeolites are 



* C. N. Fenner, " The Watchung Basalt aud the Paragenesis of its Zeolites and Other Secondary Minerals," 

 Awn. N.Y. Acad. Sci., 1910, vol. xx, pp. 93-187. 



+ hoc. cit., p. 106. 



I K. Bisz, Neues Jahrbuch fur Mincraloyic, 1896, vol. i, p. 59 ; J. S. Flett, Mem. Geol. Surv., "The Geology of 

 Newton Abbot," 1913, p. 60. 



