NCE 
NEW YORK, MARCH 16, 1894. 
ON THE ORIGIN OF ANCIENT QUARTZ ROCKS. 
BY J.-F. BLAKE, LONDON, ENGLAND. 
Tue abundance of large masses of tolerably pure 
quartz which occur in various conditions in some of the 
oldest formations is a remarkable circumstance which it 
has not yet been, I think, attempted to explain. Some of 
them may be put down in the first instance, though with- 
out exhausting our enquiry about them, as sandstones, as 
for example the “‘ original’ Huronitan quartzites and the 
Potsdam sandstone and the quartzites of Shropshire, 
Sutherlandshire; others, particularly when they are gold 
bearing, are called reefs, which may or may not be 
parallel to the stratification of the surrounding rocks. 
These are well known to characterize the older forma- 
tions in all parts of the globe. In Great Britain, as at 
Connemara in Ireland and Schiehallion in Scotland, there 
are large nearly isolated masses of white quartzite in 
Precambrian rocks, and, elsewhere, particularly in that 
portion of the series which I have called Monian, are 
numerous, discontinuous masses of quartz, as in the Sugar 
Loaf and Holyhead. There are also, in Anglesey, some 
still more isolated masses in the shape of quartz-knobs— 
small outstanding humps of white quartz rock—in the 
midst of a vast area of schists or ancient tuffs. It is the 
study of these that has suggested a possible source for a 
large part of the ancient quartz rocks and quartzites. 
Whatever difficulties may have to be got over with 
regard to the crystalline or colloidal form of the sub- 
stance, it is certain that from a chemical point of view 
the quartz of all sedimentary rocks must be derived from 
such original sources as produce practically pure silica. 
Vhere are, I think, only three such, viz.: igneous rocks, 
one of whose mineral constituents is quartz, quartz 
veins and siliceous springs. ‘The first of these has 
hitherto been almost the only source considered. ‘To get 
the grains of quartz out of igneous rocks the other 
minerals have to be separated, and where this is done with 
such exceptional completeness, as in the case of a white 
quartzite, some unusual facilities must be supposed. 
Quartz veins are an obvious source of quartz pebbles, and 
when the latter are large, as in many Cambrian con- 
glomerates, they seem necessarily derived from this 
source. As, however, quartz veins are so intimately con- 
nected with siliceous springs, both being the result of crys- 
tallization or deposit from water carrying silica in solu- 
tion, they need not be considered separately. 
The point, therefore, that I here suggest is that the 
deposits from ancient siliceous springs are an important 
source of the quartz of whichsome of the ancient quartz- 
ites and quartz rocks are composed. ‘The starting point 
of this theory is undoubtedly the structure of the quartz- 
knobs of Anglesey. In that island there are scattered 
over the surface amongst the most ancient rocks, but not 
specially related to any particular part of them, a num- 
ber of white glistening bosses of rock, which look in the 
distance like a whitewashed cottage only that they are 
usually somewhat larger. Most of these when examined 
microscopically show some rounded grains, and they 
might therefore be mistaken for ordinary quartzites. 
Their peculiar mode of occurrence, however, calling for 
more careful study, it is seen that in all there is also pre- 
sent a different structure, which in some belongs to the 
bulk of the rock or even to the whole. It is what I have 
called a polysonal structure. The whole area of the 
slide is divided up into amesh work by clear lines, the 
. interior of the polygons thus produced being spotted over 
by the minute inclusions common in quartz, which are ar- 
ranged to a certain extent in relation to the edges of the 
polygons. (Fig. 1.) Under polarized light it is seen that 
each polygon is a single crystal, whose crinkly or 
‘<sutural ” outlines so interlock with those of the adjacent 
erystals that they could only have formed in situ. (Fig. Da) 
In fact, except for all the crystals being quartz, the 
structure resembles the granitic. Where such a structure 
is in small proportion to the whole it might be considered 
secondary, but where elsewhere the bulk of the rock is of 
this kind we cannot so consider it. If, however, the sev- 
eral crystals were to separate along these polygonal lines, 
they might easily be rounded into the pebble form. There 
is, therefore, no objection that I can see to considering 
the crystalline’ portion the primary, and the rounded 
grains the secondary structure. This conclusion is con- 
firmed by certain peculiarities in the mode of occurrence. 
Thus in one example, which shows the polygonal structure 
with the greatest clearness, as we approach the knob from 
a distance we find the surrounding schists first veined 
with quartz, then the quartz veins become more abundant, 
then predominate over the schist, and finally at the knob 
itself nothing is present but the pure white quartz full of 
minute bubbles. ‘Thus the knob is intimately connected 
with veins and has no relation to any bedded rock. In 
another case there is a kind of rock crossing the stratifica- 
tion of the schists and connected at the top with a once 
horizontal mass which is parallel to the stratification. 
These cases I take to be illustrations of the pipes of the 
siliceous springs. 
Looking at the matter from the other side there seems 
no good reason to believe that such siliceous deposits as 
those of the Yellowstone Park and of Rotomahana 
should be confined to our own epoch, and yet where are 
