155 



dlesex County, the gneiss all dips to the W.N.W. ; the amount 

 of the dip varying from 50° or less to vertical. There are, as is 

 clearly shown on the map, three N.— S. bands of mica slate in 

 the Nashua Valley, each of which dips steeply to the west. 

 Between the first and second of these belts, reckoning from 

 east to west, is a long and narrow range of granite and grani- 

 toid gneiss ; while a broad band of argillite, which has also 

 steep westerly dips, separates the second and third belts of mica 

 slate. Between the first range of mica slate and the granite, 

 in Harvard and Bolton, lies the narrow strip of conglomerate 

 and argillite — conglomerate on the east and argillite on the 

 west — already described under the head of argillite. This, 

 like all the preceding, dips to the west. West of the third 

 range of mica slate is a broad expanse of gneiss, which, along 

 its eastern border, dips gently, 15°-25°, to the east ; and 

 resembles, in its essential lithological features, the gneiss of 

 Middlesex County. 



From Concord to Leominster, as just explained, a steep, 

 westerly dip is almost universal ; and, apparently, this great 

 breadth of strata, from the oldest gneiss to the newest 

 argillite, passes beneath the, lithologically, equally ancient- 

 looking, but nearly horizontal, gneiss of the Wachusett 

 range. It would be difficult to believe that the argillite is 

 older than the Wachusett gneiss, and this consideration, 

 together with the fact that the mica slate occurs on both 

 sides of the argillite, long ago suggested the notion that 

 these rocks form a huge, closed, synclinal fold with its axial 

 plane dipping to the west. According to this view, the broad 

 band of argillite is a stratum, apparently not less than 10,000 

 feet thick, folded sharply upon itself, forming the centre of the 

 synclinal. Enclosing the argillite is a stratum of mica slate, 

 forming, by its outcrops, the second and third bands of that rock ; 

 while the gneiss on the east of the valley is stratigraphically 

 equivalent to that on the west, forming the outermost and deep- 

 est layers of the synclinal. At the contact of the third band of mica 

 slate and the gneiss on the west, there is, as already indicated, 



