139 



slate on the east ; and yet the conglomerate contains not a 

 single pebble of either of these rocks, or of any rock known to 

 exist in its immediate vicinity ; but, on the contrary, the peb- 

 bles appear to have been derived from a gray quartzite, and a 

 slaty rock inclining to argillite rather than mica slate, the quartz- 

 ite pebbles predominating. Some ten miles to the south-west of 

 Harvard, however, in Clinton and Boylston, Mr. Burbank has 

 discovered extensive beds of slate and interstratified quartzite, 

 which closely resemble the materials of the pebbles that form the 

 mass of the conglomerate. This slate is the mica slate of the 

 second or middle band. The conglomerate is especially interest- 

 ing on account of the extensive and remarkable alteration that 

 it has experienced at most points. The metamorphism is best 

 shown in Harvard, near the village, where the rock is well 

 exposed and has been extensively quarried. In many cases the 

 pebbles have been flattened, bent, and even drawn out into 

 lenticular layers, developing a schistose structure in the rock. 



Limestone. — The Montalban limestones in Massachusetts 

 are rarely serpentinic, and are probably less magnesian than 

 those of the Huronian system ; though this is not very clearly 

 shown by the analyses made by Prof. Hitchcock. 1 The lime- 

 stone occurs in numerous, small, somewhat lenticular beds, 

 usually interstratified with gneiss and mica slate. The deposits 

 east of the Nashua Valley are regarded by Mr. Burbank, with 

 apparently good reason, as partaking mainly of the nature of 

 veins, rather than stratified beds ; but, west of the Connecticut, 

 as Prof. Hitchcock has pointed out, 2 some at least of the de- 

 posits are distinctly and regularly stratified. And evidence of 

 stratification is certainly not wanting in the limestone of 

 Smithfield, R.I. Of course where occurring as veins the 

 limestone deposits must be newer than the enclosing rock, 

 and may not be of Montalban age at all. The limestone 

 of Chelmsford and Bolton has afforded specimens of the 

 Eozoon canadense; and, since this peculiar structure has also 



i See Final Report on the Geol. of Mass., pp. 80-81. 

 2 Ibid., pp. 562, 566-7. 



