138 



cester. The argillite of this large area is usually fine-grained, 

 homogeneous, and black, and smooth and lustrous on the sur- 

 faces produced by division planes. According to Mr. Bur- 

 bank, the bedding of the argillite is very obscure, and it 

 appears for the most part to be much contorted ; while veins 

 and lenticular masses of quartz are not of infrequent occurrence. 

 In Lancaster and Shirley, however, he says some of the beds 

 are quite regular and the lamination smooth and even, forming 

 a good roofing slate. The most interesting variety of the ar- 

 gillite is that containing chiastolite. This mineral, as is well 

 known, is very abundant in the argillite of Sterling, Lancaster, 

 and Clinton ; and it not improbably occurs at other points in 

 the valley. In Harvard, as already observed, there is a break 

 in the second or middle band of mica slate, and here, as is 

 distinctly shown on the map, the argillite lies in contact with 

 the Worcester and Dracut range of granite. Crossing the 

 granite at this point, we find along its eastern border, wedged 

 in between it and the first band of mica slate, a narrow strip of 

 argillite and conglomerate, — argillite on the west and con- 

 glomerate on the east. This small mass, almost too small to 

 be accurately represented on the map, constitutes the most 

 interesting geologic feature of that region. It has been care- 

 fully studied in its distribution and relations by Mr. Burbank, 

 to whom I am mainly indebted for the following facts. The 

 argillite and conglomerate composing this area are distinctly and 

 conformably interstratified, although, as just stated, the argil- 

 lite is found mainly on the west side of the belt, and the con- 

 glomerate on the east. The dip is N.W. and W. 45°-80° ; 

 so that the conglomerate clearly underlies the argillite. The 

 latter rock appears to be limited to the northern and broader 

 end of the belt, and is lithologically indistinguishable from the 

 argillite of the large area one mile to the westward. The con- 

 glomerate begins in Harvard, near Harvard Centre, with a 

 breadth of less than five hundred feet, and extends in a south- 

 south-west direction to the northern border of Berlin. As 

 already stated, this belt is met by granite on the west and mica 



