CHERT FORMATIONS OF NOTRE DAME BAY 581 



All sections show more or less cloudiness at low magnification. 

 At very high magnification the impurities can be distinguished as 

 specks. They are in the form of plates and appear to have a faint 

 green color. The index is well above quartz. Under crossed 

 nicols most grains cannot be distinguished. One specimen of this 

 variety of chert was analyzed. It contains more than the usual 

 amount of impurities and was selected in the hope of shedding 

 light on their nature. The analysis is shown on page 580. 

 Some of the larger grains of the green mineral in this specimen show, 

 in addition to the above-mentioned properties, a distinct pleochroism 

 and when seen on edge a moderate birefrigence and an extinction 

 nearly or quite parallel. This analysis throws some light on the 

 mineralogic composition of the chert. Most important is the 

 amount of ferric iron. , If this were not combined the rock would 

 surely have a red color. The presence of a ferrous-ferric silicate 

 seems clear. It may be noted that with the union of similar 

 bases the ratio AI2O3 : FeO : NajO is almost precisely 5:3:1. 

 However, as the amount of all the minor constituents is so small no 

 great weight is to be attached to this. It is possible that the 

 alkaUes may be adsorbed by the silica. Their quantity is much 

 less than the amount required by glauconite and the amount of 

 alumina also points away from that mineral. There seems to be, 

 however, Httle doubt that there is present an alumina-ferric- 

 ferrous-silicate with probable alkaHes. Assuming 5 molecules of 

 silica and 6 of water, there would be present about 19 per cent by 

 weight or 16 per cent by volume of a mineral having a density 

 about equal to thuringite. This quantity, which is of course only 

 a rough approximation, seems consistent with observation, though 

 the grains are too small and their boundaries too iU-defined for 



exact measurement. 



Red Chert 



Thin sections of red chert are for the most part opaque. Some 

 sections are entirely so, but others are not uniformly opaque. In 

 these last the opacity is seen to be caused by extremely minute 

 red specks of iron oxide. The distribution of these is not uniform 

 but is without system. Most of the area is opaque, the more 

 translucent portions being in the form of clouds within the opaque 



