142 
we to find these representatives in more ancient periods 
if not in such quartz masses as these? There is this dif- 
ficulty, however, that the sample of the white terrace of 
Rotomahana that I have examined has no action upon 
polarized light and is not, therefore, crystalline. But 
this difficulty is easily explained by comparison with the 
glassy form of igneous rocks which does not prevent us 
looking on other rocks as igneous which are crystalline. 
An example of a pure quartz vein cutting like a dyke the 
igneous rocks of Chamwood Forest, Leicestershire, which 
I have examined, shows a microcrystalline structure, very 
like that of some felsites, especially the older ones. The 
association of rounded grains of quartz should also no 
more surprise us than the association of volcanic ashes 
with eruptive crystalline rocks and association so intimate 
in the case of some of the Italian volcanoes that it is 
Fig. 1. Structure of a quartz knob in Anglesey 
seen under ordinary light. 
Fig. 2. Structure of the same seen between 
crossed nicols. 
sometimes difficult to say where one begins and the other 
ends; for instance, on the island of Ischia I have observed 
an obsidian crowded with fragments. By this comparison 
I would not be understood to imply that these quartz 
knobs are ‘‘igneous,”” but I think they are of hydro- 
thermal origin, as to some extent also are true igneous 
rocks. 
With regard to the larger masses, if they are not of the 
nature of quartz veins, as many of the gold bearing reefs 
may be, they are probably derived by attrition, etc., from 
original siliceous deposits of the same nature as the above, 
and in this case they are not fragments of a dislocated 
bed of continuous sandstone, but mark the proximate 
sites of previously existing siliceous springs,—the sum 
total of the deposits from such bearing naturally no pro- 
SICIUBINC!S, 
‘colored plates: 
Vol. XXIII. No. 580 
portion to the size of the rocks up which they have been 
brought, but being of far greater size. _Even the con- 
tinuous beds of quartzite having an ordinary stratified ar- 
rangement may be derived in the first place from such 
sources, which will in part account for their whiteness. 
There are one or two subsidiary observations which 
lend some support to this contention. As’ originally 
noted by Dr. R. D. Irving, the quartz grains of the 
Huronian quartzites are cemented by an additional 
growth of quartz around them, and though this quartz is 
of course of secondary origin, it shows that siliceous 
water was percolating rocks inthe district. But of more 
importance is the occurrence* of pebbles of jasper, and 
large ones of pure vein quartz in these Huronian quartz- 
ites. The former mineral is: not an original constituent 
of quartz bearing plutonic rocks, but it is undoubtedly 
formed in the wet way. Similar jasper pebbles are formed 
also in Anglesey in association with limestones where 
these are themselves associated with the quartz rocks. 
There are also large tabular crystals of platz haematite in 
the white Potsdam quartzite near Philadelphia. ‘Thus 
the associates of the quartz in sach quartzites are not 
those common in granites, ete., but those which point to 
aqueous agencies. 
Again, though more rarely. 
ciated with the quartzites which have the same pecu- 
liarities of distribution and mode of cccurrence. ‘These 
show no traces of organic structure, though there is no 
reason that I can see why it should not be present even 
in Pre-Cambrian rocks, but in some cases they do show 
very decided traces of a tufaceous origin. One example 
in Anglesey being especially remarkable, as it consists of 
a compound oolite, layer after layer of irregularly de- 
posited calcite forming the coats, and a pair of the 
smaller bodies being sometimes surrounded by other coats 
embracing the two. ‘There are also lenticular patches of 
bedded limestone probably like the larger quartz masses 
derived from these—a similar patch of tufaceous-looking, 
non-organic, limestone occurs in the Huronian series north 
of Lake Huron. 
For these various reasons it seems to me that we must 
at least consider the possibility of the original source of 
the quartz in these early quartzites being the deposit 
from siliceous springs, possibly from the more crystalline 
character, bursting out under somewhat different condi- 
tions as to pressure, etc., from modern ones, and from 
the abundance of such quartzites we may look upon the 
later Pre-Cambrian period as characterized by the 
abundance of such springs, an idea not at all inconsistent 
with the supposed volcanic origin of many of the so-called 
Archean rocks, 
we find limestones asso- 
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