Graywcke Breccias. 



255 



of them are not less than a foot wide; and they are frequently- 

 branched. These veins separate the imbedded nodules, and are 

 chemically united to the divided portions. These veins and the semi- 

 crystalline aspect of the cement of this rock, prove it to be one of the 

 oldest of the varieties which I have included under the term gray- 

 wacke. 



2. Breccias. These are distinguished from the conglomerates by 

 the angular shape of the imbedded fragments. One variety (Nos. 

 296, 297,) consists of fragments of reddish and ash colored argilla- 

 ceous slate, united by an argillaceous or arenaceous cement. This 

 aggregate is slaty, and the cement has a porphyritic appearance. I 

 have observed it only in a few places ; as at Natick and Randolph. 

 Another variety, approaching to slaty porphyry, appears to be com- 

 posed chiefly of compact feldspar, united by a cement of comminuted 

 porphyry. This was found also in Natick. (No. 298.) A third va- 

 riety, (No. 299,) of which I found only a bowlder in Saugus, consists 

 of gray compact feldspar (?) and indurated wacke. (?) A fourth 

 consists of fragments of gray and yellowish green compact feldspar, 

 united by an unknown dark colored cement. (No. 300.) The yel- 

 lowish green variety appears as if colored by epidote. This most 

 singular rock occurs at the head of Nantasket Beach, in Cohasset ; 

 and when its ledges are moistened by the spray, they present a most 

 fantastic and really a very splendid appearance, resembling exceed- 

 ingly variegated serpentine. The rock exhibits no regular strata, 

 although divided like the unstratified rocks generally by numerous 

 seams. It is associated with a conglomerate, similar to the variety 

 first described above, and I suspect it to be the rock marked in Co- 

 hasset on the map of the Messrs. Danas as ' Petrosilex.' If large 

 blocks can be obtained and it will admit of being polished, it will 

 furnish an elegant ornamental stone. 



The conglomerate just, mentioned, as associated with the breccia at 

 the head of Nantasket Beach, extends into Hingham ; and both in 

 Cohasset and Hingham, it assumes a character intermediate between 

 puddingstone and breccia, by taking into its composition angular and 

 partly rounded masses of poryhyry, greenstone, and amygdaloid. 

 These are sometimes so numerous, that the rock might easily be mis- 

 taken for a variety of trap. Neither this rock nor the conglomerate, 

 of which, indeed, it forms a part, exhibit, so far as I could perceive, 

 any evidence of stratification : although being a rock of mechanical 

 origin, it is undoubtedly stratified. 



