240 The American Geologist. Apt, i 
As additional evidence that the quartz in the greenstone 
may be secondary rather than residuary may be mentioned 
the crystalline quartz occurring with the siderite, chalcodite, 
millerite, etc., in the pockets of the hematite; and the fact, well 
attested by specimens in my collection, that the greenstone has 
been metasomatically replaced by quartz on a considerable 
scale. One specimen shows the complete replacement of an 
angular joint-block of the greenstone nearly a foot long and 
half as thick, forming a sharply angular quartz geode of these 
dimensions (fig. 4). Even granting that the granular quartz 
lic, 4.—An angular quartz geode, due to the replacement of a jOint-block of 
the chloritic rock. About one-third natural size. 
is partly or wholly residuary does not prove the deriviation of 
this ultra-basic rock from the granite, for the original basic 
rock of this dike may «very well have been quartziferous, a 
quartz-dioryte, or possibly a quartz-gabbro. The original rock, 
while quite certainly not so acid as the granite, was not neces- 
sarily highly basic; and the magmatic segregation of sulphides 
is by no means confined to rocks eXceptionally low in silica; 
but they are fairly common in rocks of neutral or sub-acid char- 
acter, and even, as in the case of some of the ore-bearing 
