ON THE ORIGIN OF ANCIENT QUARTZ ROCKS

J. F. Blake
1894 Science  
T H E abundance o f large masses o f tolerably pure quartz which occur in various conditions in some o f t h e oldest formations is a remarkable circumstance which it has not pet been, I t h i n k , attenlpted t o explain. Some o f t h e m may be put down in the first instance , though without exhausting our enquiry about t h e m , as sandstones, as for example t h e " original " IIuronian quartzites and the Yotsdain sandstone and the quartzites o f Shropshire, Sutherlaadshire; others,
more » ... e; others, particularly when t h e y are gold bearing, are called reefs, which may or may not b e parallel t o t h e stratification o f t h e surrouilding rocks. 'l'hese are well known t o characterize t h e older formations in ail parts o f the globe. In Great Britain, as at Connemara in Ireland and Schiehaliion in Scotland, there are large nearly isolated masses o f white quartzite in IJrecambrian rocks, and, elsewhere, particularly in that poi-~ion o f the series which I have called Monian, are numerous, discontiiluous inasses o f quartz, as in t h e Sugar I,oaf and Holyhead. There are also, in Anglesey, some still more isolated masses in the shape o f quartz-knobssinall outstanding humps o f white quartz rock-in the rnidst o f a vast area o f schists or ailcieilt t u f f s . It is the study o f these that has suggested a possible source for a large part o f the ancient quartz rocks and quartzites. IVhatever difiiculties may have t o be got over with regard t o t h e crystalline or colloidal f o r m o f the substance, it is certain that from a chemical point o f view t h e quartz o f all sedimentary rocks must be derived f r o m such original sources as produce practically pure silica. 'l'here are, I t h i n k , only three such, viz. : iglleous rocks, one o f whose mineral constituents is quartz, quartz veins and siliceoi~s springs. 'The first o f these has hitherto been almost t h e only source considered. T o get t h e gr,ains o f quartz out o f igneous rocks t h e other minerals have t o be separated, and where this is done with such exceptional completeness, as in t h e case o f a white quartzite, some unusual facilities must be supposeti. Quartz veins are an obvious source o f quartz pebbles, ant1 when t h e latter are large, as in many Cambrian conglomerates, they seeill necessarily derived from this source. A S , however, quartz veins are so intimately connected with siliceous springs, both being t h e result o f crystallization or deposit froni water carrying silica in solution, t h e y need not be considered separately. 'I'he point, therefore, that 1 here suggest is that t h e deposits from ancient silicefous s p r i n g s a r c an irnportnnt source o f t h e quartz o f which some o f the ancieilt quartzites and quartz rocks are composed. T h e starting point o f this theory is undoubtedly t h e structure o f the quartzknobs o f Anglesey. Iri that island there are scattered over the surface amongst t h e most ancient rocks, but not specially related t o any particular part o f t h e m , a number o f white glistening bosses o f rock, which look in t h e distance like a whitewashed cottage only that they are usually somewhat iarger. Most o f these when examined inicroscopically show some rounded grains, and they might therefore b e mistaken for ordinary quartzites. Their peculiar mode o f occurrence, however, calling for inore careful study, it is seen that in all there is also present a different structure, which in some belongs t o t h e bulk o f the rock or even t o t h e whole. I t is what I have called a polysonal structure. T h e whole area o f t h e slide is divided up into a mesh work b y clear lines, t h e interior o f the polygons thus produced being spotted over b y the minute inclusions c o m m o n in quartz, which are arranged t o a certain extent in relation t o t h e edges o f t h e polygons. (Fig. I . ) Under polarized light it is seen that each polygon is a single crystal, whose crinkly or ( ( sutural " outlines so iilterlock with those o f t h e adjacent crystals that t h e y could only have formed in situ. (Fig. 2 . ) In fact, except for all t h e crystals being quartz, the structure resembles the granitic. Where such a structure is in small proportion t o the whole it might be considered secondary, but where elsewhere t h e bulk o f t h e rock is o f this kind we cannot so consider it. I f , however, the several crystals were t o separate along these polygonal lines, they might easily b e rounded into t h e pebble form. T h e r e is, therefore, no objection that I can see t o considering t h e crystalline portion t h e primary, and t h e rounded grains t h e secondary structure. T h i s conclusioil is confirmed b y certain peculiarities in t h e mode o f occurrence. T h u s in one example, which shows t h e polygonal structure wlth the greatest clearness, as we approach the k n o b froin a distancr we find t h e surrouilding schists first veined with quartz, t h e n the quartz veins become more abundant, then predominate over the schist, and finally at t h e k n o b itself nothing is present but t h e pure white quartz full o f minute bubbles. T h u s the knob is intimately connected with veins and has n o relation t o any bedded rock. I n another ca5e there is a kintl o f rock crossing t h e stratification o f t h e schists and coilnected at t h e t o p with a once h o r i~o n t a l mass which is parallel t o the stratification. 'I'hese case.; I t a k e t o b e illustratioils o f t h e pipes o f t h e siliceous springs. 1,ooking at t h e matter f r o m the other side there seems no good reason t o believe that such siliceous deposits as those o f t h e Yellowstone Park and o f Rotolnahai~a shoultl he confined t o o i~r0~~1 1 epoch, aud yet where are,
doi:10.1126/science.ns-23.580.141 pmid:17753030 fatcat:zxphug2uuvhnpi5e4simzbf75e