I always knew that Flynt was consisdered a granite quarry, until I started my research. I didn't even know what gneiss or specifically Monson Gneiss was.

As the chart shows that Granite is a igneous (volcanic) rock & Gneiss is a foliated metamorphic rock, apparently formed at great depths (at least at high pressure) & heat.

Apparently, the two are sometimes found in close proximity, such as I suspect in Monson. I also assume that Monson Gneiss derives the classification from the Flynt Quarry.

From: A Mineralogical Lexicon of Franklin, Hampshire, and Hampden Counties

Gneiss differs from granite in that its various constituents are arranged in more or less paralled layers, giving the rock a banded or schistose structure and causing it to split in a direction paralled with the bands much more readily than across them. Chemically and from a mineral standpoint the rock is identical with granite and is used so far as these structural peculiarities will allow for the same purposes. It is therefore in the following pages included with the granites. The word gneiss, it should be stated, is of German origin and pronounced as is our word nice, not as though spelled nces. These rocks are often called stratified or bastard granites by the quarrymen.

Frankly, I think 'Gneiss Granite' sounds better (nicer) than 'bastard granite', particularly if we were exporting it.