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miles, extends south-west through Westford and Littleton to 

 Harvard, and thence with a diminished and uncertain breadth 

 to Worcester. The northern band becomes very contracted in 

 Dracut, and then widens somewhat, passing along the north 

 side of the granite through Tyngsborough and southern Dun- 

 stable to Groton, where it joins a great body of the same 

 rock, which, trending in a more nearly north and south direc- 

 tion, extends south between the granite and the argillite on 

 the west, becomes rapidly narrower, and disappears in Har- 

 vard. It reappears in Bolton, and stretches thence, one to two 

 miles wide, to Worcester, where it reunites with the first band, 

 about thirty-five miles from their point of separation. A third 

 band of mica slate crosses the New Hampshire line in Town- 

 send, and, widening rapidly, spreads southward through Lunen- 

 burg and Fitchburg, where it is six miles wide ; then, narrowing 

 gradually, it continues through Leominster and Sterling to 

 Worcester, where it is joined by the other two belts, and 

 stretches on thence with an average breadth of about three miles 

 through Auburn, Oxford, and Webster to the Connecticut line. 



The rock composing the irregular area outlined above is sub- 

 ject to extensive variations, and comparatively little of it can 

 be regarded as typical mica slate. It is usually less crystalline, 

 and frequently less micaceous and more argillaceous, than the 

 mica slate beyond the Connecticut. The quartz is often the 

 predominant constituent; and in some places, notably in Clin- 

 ton and Boylston, numerous bands of a distinct quartzite are 

 interstratified with the slate. 



Argillite. — Argillite of several distinct ages occurs in 

 Massachusetts ; but to the Montalban age is referred the argil- 

 lite of the Nashua and Connecticut Valleys only. A broad 

 belt of argillite occupies the centre of the Nashua Yalley, lying 

 between the second and third bands of mica slate. It enters 

 the State on the north with a breadth of four miles ; widens 

 gradually southward, attaining its maximum breadth of six 

 miles in Lancaster and Harvard ; whence it stretches south- 

 south-westerly, with constantly diminishing breadth, to AVor- 



