On the Fissure-Iticlusions. 



115 



tain otherwise invisible planes. Evidently, that careful ob- 

 server, Sorby, was on his guard in this respect, as he makes the 

 following statement concerning the fluid cavities in the quartz 

 of quartz- veins : 



"Sometimes we may distinctly see that the quartz has been 

 cracked, and the cracks afterwards filled up with quartz. This 

 .... appears in some cases to have taken place at alow tem- 

 perature and explains why bands of cavities occasionally occur 

 with vacuities relatively less than those in the fluid cavities of 

 the general mass." I believe, however, that in some instances, 

 as in this very rock, fissure-inclusions of this class are quite 

 difficult to identify, from either their sparseness or their great 

 number and irregularity. It may be further suggested that 

 since such inclusions appear to have been formed during the 

 folding or extrusion of a mass, they present the exact con- 

 ditions for the excess of pressure indicated in the calculations 

 of Sorby and Ward, and by them attributed to the same cause 



Again, the distinctions between rocks founded upon the pres- 

 ence or abundance of fluid-inclusions in certain groups, such 

 as that proposed by Zirkel* between the metamorphic and 

 younger eruptive granites of the West, on account of the fre- 

 quent poverty of the former in liquid-inclusions, may, although 

 local, yet meet with striking exceptions and need careful recon- 

 sideration. In the case of this metamorphic gneiss of West- 

 chester and New York counties, w^hich is certainly a member 

 of the Montalban group, such a scheme, if pressed to a conclu- 

 sion, would conflict with the idea of Archean age, and rather 

 tend to corroborate the current suspicion, on other grounds, 

 of its Silurian origin. 



Finally, it may be briefly stated that the same fissure-inclu- 

 sions in planes have been commonh' observed in the quartz- 

 grains of American sandstones, terminating abruptly at the 

 outlines of the grains. They occur also in the quartz-grains 

 of many metamorphic or conglomeritic gneisses of Wisconsin, 

 North Carolina, Colorado, f etc., and with the same abrupt ter- 

 minations at the limits of the grains, which, as Zirkel observes 

 of the last locality, " makes them appear like worn elastic 

 fragments." 



*Zirkel, loc. cit., 55. 



tExpl. of 40th Par. VI., 36, 58. 



