SILICIFICATION. 219 
small amount of ferration, the result of which is seen in the appearance of 
oceasional hexagonal crystals of specular iron and, almost invariably, 
disseminated limonite. Often limonite occurs i numerous small rhombo- 
hedra, which are probably pseudomorphous after siderite, the siderite 
having probably formed from alteration of the original calcite. 
The first stage of silicification is, as before noted, the appearance of 
isolated crystals of quartz in the limestone. A similar phenomenon has 
been noticed in most of the other metasomatic changes, such as ferration 
and ore deposition in general. The initiation of these processes is signal- 
ized by the occurrence of isolated, perfect crystals in unaltered rock, and 
these crystals enlarge or multiply until they jom, and thus accomplish a 
complete replacement. 
The rock that results from the complete silicification of limestone or 
dolomite sometimes resembles in the hand specimen a chert, but more 
often a fine-grained and altered quartzite. It is generally somewhat 
porous, its porosity arising from the leaching out of residual calcite areas, 
and it is usually drusy, with coatings of crystalline and chalcedonic 
quartz. Its color is usually red or yellow from the iron which it contains. 
This rock is one which is often found developed on a large scale in many 
mining districts, and has been studied by the writer at Leadville, where 
it occurs extensively in connection with the ore deposits. Its structure, 
appearance, and general method of origin are practically identical with 
the famous jasper of the Lake Superior iron regions. 
During a study of the iron-bearing rocks of the Mesabi range in Min- 
nesota’ the writer pointed out that the word “jasper” was not a strictly 
correct name for this peculiar quartz rock, but concluded to retain it for the 
iron-bearing series. Since that time, however, he has become impressed 
with the widespread occurrence of this variety of quartz, which arises from 
the replacement of some original rock, ordinarily limestone or dolomite, by 
silica from circulating waters; and there seems to be need of some term 
which may specifically dicate it. For this use the word ‘‘jasperoid”? is 
suggested. Jasperoid may then be defined as a rock consisting essentially 
of cryptocrystalline, chalcedonic, or phenoerystalline silica, which has 
formed by the replacement of some other material, ordinarily calcite or 
1 The Mesabi iron-bearing rocks: Bull. Minnesota Geol. Survey No. 10, p. 135. 
» Meaning a jasperlike rock. 
