

of Boston and its Vicinity. Sll 



II. Quartz and Petrosilcx are the most abundant ingredients ; 

 the former presents various colours ; it is sometimes translucent, 

 and the smaller grains are transparent ; bul generally it is opaque, 

 with a splintery and imperfectly granular, or compact fracture. 

 All the colours mentioned in the mineralogical part of this work, 

 as belonging to Petrosilex, occur in the nodules, which are found 

 in Grey wacke, and often, some of the most beautiful varieties are 

 contained in this rock. From its great liability to decomposition, 

 Petrosilex is generally one of the first portions of Greywacke, 

 which begins to decay ; and it is sometimes so much altered, as 

 to resemble indurated clay. The Porphyry has a base of reddish 

 Petrosilex, including small crystalline grains of Feldspar, and 

 sometimes of Quartz, but it forms a small portion only of the in- 

 gredients of Greywacke. The Argillite will be immediately 

 recognised by its bluish slate colour, and structure. The Sienite 

 contains transparent grains of Quartz, a small proportion of Horn- 

 blende, with crystalline grains of Feldspar, whose colour is 

 either red or grey, with a foliated fracture and a waxy lustre. It 

 rarely contains Epidote. 



III. The nodules are not cemented by a basis of Argillite, as 

 is frequently the case in Greywacke, but they are united by a 



finer grained Greywacke. The interstices between the large 

 . nodules are filled with very fine Greywacke, which sometimes 



■ ^ 



appears almost homogeneous^ from the very minute grains of 

 wbicli it is composed. 



IV. Greywacke is traversed by small grains of Quartz; and 



Greenstone and Argillite form extensive beds in it, at Brookline^ 

 Brighton; and Newton. Amygdaloid is intimately connected 







