356 



Dresser — Geology of Brome Mountain. 



The norm calculated from this analysis is as follows 



Orthoclase 31'69 



Albite 27*77 



Nepheline _ 25*56 



Acmite _ _ 2'31 



Diopside ._ 7*85 



Olivine _ "24 



Ilmenite 1*06 



Magnetite T86 



Water 1*50 



99-84 



It is therefore classed as follows — . 



Class II, Dosalane 



Order 6, Norgare 



Rang 1, Laurdalase 



Subrang 4, Laurdalose 



The structure of this rock is both macroscopically and 

 microscopically porphyritic. 



As sodalite is one of the few distinguishable minerals in it, 

 and is indicative of its alkaline character, it might best be 

 designated as a sodalite-bearing felsophyro-laurdalose. 



Comparison of Brome and Shefford. 



When* compared with Shefford Mountain the similarity of 

 the two hills is found to be very close. In Shefford Mountain 

 there have been three separate eruptions and the rocks thus 

 produced in order of intrusion are very similar to those of 

 Brome. 



Shefford 

 1st, Akerose (Essexite) ; 

 2d, Nordmarkose (Nordmarkite) 

 3d, Laurvikose (Pulaskite) 



Brome 

 1st, Hessose 

 2d, Nordmarkose 

 3d, Laurdalose 



The rocks of the first intrusions in the two hills thus corre- 

 spond very closely while those of the second are identical, 

 while the third classes do not differ widely. 



In point of general structure the hills are practically alike 

 except that dikes later than the main mass of the mountain 

 are very numerous at Shefford while they are almost altogether 

 wanting at Brome. 



Chemical Composition of Original Magma. 



An effort was made to ascertain the chemical composition of 

 the original magma which produced these rocks. The present 

 surface exposure of the mountain may be taken as affording an 

 average cross section of the mass. Accordingly, the relative 



