O. A. Derby—Geology of the Diamond. 97 
height above low water in the river of 140 feet to that of only 
5d feet; that it has this low level, continued northward, 
with little change, for eighteen miles, or more than half way to 
the mouth of the Passumpsie; and that at Haverhill, seventeen 
miles north of Thetford, the “highest normal terrace” is 
135 feet. But, taking the terraces at the mouths of tributaries 
as marking the flood-level along this part of the valley—the 
so-called ‘“ delta-terraces” of Oliverian Brook, Jacob’s Brook, 
Kastman’s Brook and others—and then the low terraces 
referred to become simply the lower terraces along a part of 
the valley in which the highest normal terraces are wanting, 
except at the mouths of tributaries.* 
With regard to the high terrace-plain of Haverhill there are 
other facts besides those connected with the terraces of Oliver- 
lan Brook ; and these are presented on a following page. 
[To be continued. ] 
Art. IX.— Geology of the Diamond ; by O. A. DERBY. 
na paper by myself, published in vol. v of the Archivos do 
Museu Nacional of Rio de Janeiro, the famous locality of Grado 
Mogol, in which diamonds are found in quartzite, is discussed. 
It is shown that, under the name of itacolumite, two very dis- 
tnct geological series have been confounded. The older series, 
Including the true flaggy and often flexible quartzite to which 
the name should be limited, is intercalated with the unctuous 
(hydro-mica) schists and itabirites. The newer series is com- 
posed almost exclusively of quartzite, which in its finer parts is 
almost indistinguishable from true itacolumite, but which, in 
Places, passes to a conglomerate containing pebbles of all the 
rocks of the older series. Throughout the diamond region of 
the Serra do Kspivhaco this quartzite lies in well-marked uncon- 
formability on the upturned edges of the lower, though, since 
* Mr. Upham explains, (p. 79,) the existence of terraces of small height as the 
Bong irregular rate of retreat of the ice-sheet, allowing long and abundant deposi- 
‘on in some portions, ut much less in other portions of the valley.” 
