Announced by all the trumpets of the sky,
Arrives the snow, and, driving o'er the fields,
Seems nowhere to alight: the whited air
Hides hills and woods, the river, and the heaven,
And veils the farm-house at the garden's end.
The sled and traveller stopped, the courier's feet
Delayed, all friends shut out, the housemates sit
Around the radiant fireplace, enclosed
In a tumultuous privacy of storm.
Come see the north wind's masonry.
Out of an unseen quarry
Furnished with tile, the fierce artificer
Curves his white bastions with projected roof
Emerson is saying that the wind ( north wind ) is building the snow like bricks. 
 
 (Not this literally, but you get the point.) 
 If you don’t know the definition of a masonry: 
 
