BY L. A. COTTON. 805 



have become encrusted and cemented by calcite, resulting from 

 the decomposition of the basalt itself. At the latter place, the 

 diamonds have been derived from the destruction of a tributary 

 of the main lead, which connected Soldier Hill with the Round 

 Mount. 



Though a few diamonds have been won from these recent 

 deposits, by far the greater number have been recovered from 

 the Tertiary deposits. In rare cases, the basalt-capping has been 

 denuded, and the gravels lie exposed at the surface. These 

 original deposits are readily detected from the recent alluvials, 

 both by their position and the nature of their gravels. Examples 

 of such deposits are the Streak of Luck, and the Sandy Block. 

 The greater part of the Tertiary gravels, however, are now to be 

 found underlying basalt-flows at various depths from the surface. 



The age of the diamond-bearing gravels can be determined 

 only relatively to the rocks between which they lie, as no 

 definite internal evidence has yet been found. Though lignite 

 occurs in several of the mines, no definite plant-remains have 

 been obtained from this locality. 



The granites are to be correlated with those at Ashford, which 

 have been stated to intrude Permo-Carboniferous sediments. 

 The coarse grain of the granite indicates that it must have con- 

 solidated under a considerable thickness of the sedimentary rock. 

 As, however, but little of the latter remained at the time when 

 the basalt-flows occurred, a considerable interval must ' have 

 elapsed between the intrusion of the granite and the formation 

 of the deep leads. 



The evidence of fossil leaves, from other leads in New England, 

 points to a late Tertiary age for the basalts of the plateau. In 

 the lack of any more definite evidence, therefore, the Copeton 

 diamond-deposits may be regarded as of late Tertiary age. 



Tertiary yravels. — 'J'he nature of the diamond-bearing drifts 

 has obviously been determined by the character of the country 

 over which the prebasaltic streams flowed. The main lead at 

 Copeton has had its course entirely in granite. This rock is of 

 an aplitic character, and, being composed chiefly of quartz and 

 felspar, has, as its products of decomposition, quartz-grains and 



