REPORT OF THE CHIEF ASTRONOMER 301 



SESSIONAL PAPER No. 25a 



Quartz , 



Andesine 



Biotite 



Muscovite 



Magnetite 



Other accessories.. .. 



100-0 



The calculated chemical composition (biotite assumed to have the average 

 composition of average biotite in California and Montana quartz-monzonites) is 

 approximately : — 



SiOo 664 



A1 2 18-0 



Fe„0- 10 



Feb 1-6 



MgO 1-2 



CaO 4-2 



Na 2 4-8 



K.O 1-4 



^\ ater. etc 1*4 



100-0 



The specific gravity of the specimens is 2-687. The rock is a muscovite-bear- 

 ing quartz diorite. 



Relation of the Stocks to the Bayonne Batholith. — The width of the shatter- 

 belt and of the metamorphic aureole about these intrusions is out of all propor- 

 tion to the visible size of the latter. Their explanation is simple if it be credited 

 that granite underlies, at no great depth, these belts of profound mechanical and 

 mineralogical changes or, in other words, that the altered schists belong to the 

 roof of an intrusive body much larger than the whole area of plutonic rock 

 actually exposed. Following this line of argument the writer believes that the 

 granite of the larger stock represents tbe crystallized product of the uppermost 

 part of a batholithic mass. Further, the evident similarity of the dominant salic 

 components of this stock granite with those of the Bayonne granite, as -well as 

 the matching of the biotitic and minor accessory constituents in the two cases, 

 leads to the hypothesis that the stocks are not only spatially but genetically 

 satellites of the Bayonne batholith. (Figure 22.) Like the latter, the stocks are 

 quite uncrushed and cut the upturned Summit series of sediments; the exposed 

 stocks and the batholith are contemporaneous, so far as the field evidence can 

 decide. 



If it be assumed that the stocks and the exposed batholiths are really con- 

 nected underground from a greater only partially unroofed, batholithic mass, an 

 important question arises. Erosion has removed from the Bayonne body several 

 thousand feet of granitic rock, measured vertically. We are thus in a position 

 to determine the density of the mass crystallized at points thousands of feet 

 below the roof of this part of the batholith. In the Summit stock we are able to 

 determine the density of the granite crystallized very near the roof of an offshoot 

 from the same batholith. The average specific gravities are, respectively, 2-757 



