404 



THE AZOIC SYSTEM AND ITS SUBDIVISIONS. 



Prof. Hitchcock*s section is, that all the formations lying between Bos- 

 ton and a point some seventy miles west are represented aa extending 

 perpendicularly downwards. 



Later, Prof. John W. Webster published his " Remarks on the Ge- 

 ology of Boston and Vicinity.'* (The Boston Journal of Philosophy and 

 the Arts, 1824-25, IL, pp. 277-292; 1825-2G, III, pp. 48G-489.) lie 

 states that the Boston peninsula exhibits no rock in jJacc except at one 

 locality. This was a light gray clay slate (argillite) found by digging 

 to a considerable depth below the surface. The syenite seen, which had 

 led to the belief that the rock of the peninsula was syenite, had been 

 found to be a boulder. Winter Hill of Charleatown (now in Sonier- 

 ville) was said to be composed of clay slate passing on the north into 



r 



hornblende slate. The dip was towards the north, at an angle of from 

 15° to 20°. These rocks contain " beds and veins of greenstone." The 

 lowest rock on Prospect Hill, near Winter Hill, was regarded as a 

 greenish compact feldspar,* in places strongly resembling some varieties 

 of limestone. This rock passed into clay slate and dipped " to the 

 south, inclining a little to the west, under an angle varying from 20° 

 to 50°." This was overlain by trap, which it was thought once formed 

 an extensive bed covering the slate. The Granite Street diabase of 



Like- 



Prospect Hill was described and called a "sienitic greenstone. ' 

 wise the diabase of the Powder-House, Somervillo, and of Medford, is 

 mentioned, and the boulder-like disintegration pointed out. The oc- 

 currence of slate north of the Powder-House was noted, together with 

 trap overlying the slate or interposed between the strata. 



The Roxbury conglomerate was described at some length, and said to 

 pass " into coarse gran wackc, iino grained gran wacke, and grau wacko 

 slate which becomes at last distinct clay slate." The componeiits of the 

 conglomerate enumerated were hornstone, quartz, compact feldspar, 

 flinty slate passing to Lydian stone, porphyry, granite, clay slate, no- 

 vaculite, serpentine, and nephrite. The occurrence of *' trap veins or 

 dikes " in the conglomerate was noticed. The conglomerate was said to 

 pass into amygdaloid,- the latter being the overlying rock. Another 

 transition observed was in crossing the conglomerate of Dorchester, 

 which was seen to acquire " a greater degree of compactness and uni- 

 formity of composition until within about three miles of the Blue Hills, 

 where it passes into compact feldspar, and this last into hornstone." 

 The Blue Hills were said to be composed principally of a '^peculiar por- 

 phyry resembling some of the trachytes." This rock overlaid the sye- 



* This is tlio indurated argillite of that vicinity. 



