Dodge.] 392 [February 3, 



Britain, also, more careful study of many of the old "sienite and 

 greenstone" areas has shown them to contain stratified as well as 

 crystalline portions. 



In northern Topsfield and eastern Middleton, it is stated that there 

 are silicious slates, partly cherty, partly granular, like sandstone. 



In Woburn there are silicious slates, often very much contorted, 

 dipping west-north-west, in what is known as Shaker Glen, along the 

 course of Mill Brook. Above Spy Pond in Arlington, there are slaty 

 rocks which pass through fine grits to coarse sienite by various stages. 

 So, too, in Medford. There is white quartzite banded with pink, with 

 a high west or north-west dip, in Waltham; and a granular, almost 

 translucent variety in Natick with sienite. In the north-eastern 

 part of Weston, between a brook and the Lexington road, there is a 

 ledge of sandy and slaty rock dipping 60° or 70° west-north-west. 

 In most of these stratified rocks, there are frequently small epidotic 

 bands, apparently of later formation, between the layers, expanding 

 occasionally into nodules. This occurs also quite frequently in the 

 case of more recent slates lying near the crystallines in some places. 



Breccia occurs in many places. Two very easily accessible locali- 

 ties are at the end of the band of crystallines of Dedham. One of 

 these, a detached area, east of the Pine Garden Station (Boston and 

 Providence Railroad) in the northern part of Hyde Park, is sur- 

 rounded by conglomerates. The other is a mile or two south-south- 

 west; that is, about north-west of Hyde Park village. Here the 

 rock is a light colored felsite which is often compact, but in places has 

 a wavy lamination. Where this is brecciated, the fragments are of 

 both varieties of the original rock. At the first locality the breccia 

 is similar but more conglomeritic, and consists apparently of small 

 fragments of flesh-red or ash-colored slates closely massed together 

 or united with a compact felspathic paste. 



To illustrate further the lithological character of the crystalline 

 area taken as a whole, it may be well to note some of the less abun- 

 dant minerals which have been stated to occur. As has been already 

 seen, both the acid and basic types of rock occur, together with many 

 intermediate varieties combining the characters of each. Excepting 

 the hornblendic rocks already mentioned, however, the latter class, 

 well described by Dr. Hunt as " consisting in great part of the sili- 

 cates of the protoxyd bases, lime, magnesia and ferrous oxyd, either 

 alone or in combination with silicates of alumina and alkalies," 

 is not very extensively represented here. There are occasional lime- 



