THE MONSON (iXEISS. 65 



are likened by the niiuers to the iiriiig of a bhist, and have been heard a 

 mile awa}' from the quarry- 

 Professor Niles explains the j)henomena here described as the result of 

 a force of compression acting from north to south. Tliis would he a later 

 exertion of the same mountain-making- force which, acting from east to 

 west, has folded the rocks in meridional ridges. I know of no inde})endent 

 evidence of the replacement of the east-west force by si later north-south 

 compression. The changes of level in the Glacial period, and along our 

 coast in later ages, seem to come under a different category. 



The rock is certainly in a state of elastic compression in a north-south 

 direction at present, while the last traceable dynamic change it has under- 

 gone was the strong east-west pressure wliicli crushed its pebbles into flat 

 disks and caused them to be so greatly stretched in the vertical direction. 

 In the resolution of this force some portion of its north-south component 

 seems to have been stored in the rock as an elastic stress which expresses 

 itself in expansion when the surrounding masses are removed. 



A COMPLEX MINERAL VEIN OF THE GNEISS. 



A cm-ious vein occurs in the northwest corner of the Monson quan-v, 

 adjacent to the region where the gneiss is highly homblendic. The earliest 

 filling of the vein was a matted mixture of pale-green, fine-fibrous actino- 

 lite, granular to short-bladed epidote, clinochlore, magnetite in octahedra, a 

 little quartz, and colorless prehnite in thin, flat blades of the form (001), 

 00 P Ob (010), 00 P CO (100), 00 (110), flattened on and elongate parallel 

 to the short axis. They are very minute, but were determined crvstallo- 

 graphically by fixing the position <if the ojjtical axes. This ends with a 

 downy surtace of thin, colorless prehnite blades, and a second series begins 

 with calcite in fine, transparent, cleavable masses, followed by rich-green 

 prehnite in rosettes and sheaf-like forms, upon which is a final generation 

 of calcite in distinct crystals — the rhoml^ohedron R with its edges replaced 

 by a scalenohedron. 



In other parts of the vein this is followed by laumontite, at first inter- 

 grown with the prehnite and then resting upon it. It is in fine, large crys- 

 tals and coarse-granular crystalline aggi-egates of pink-white color. The 

 series is closed by a leek-green hornstone, which fills the vein and envelops 

 the laumonite. 



MON xxix 5 



