164 T. Crook—Titaniferous Volcanic Rocks. 
modern petrographer stands to gain by a sympathetic consideration of 
the methods which he used. By way of example, we may again refer 
to Dr. Washington’s work on ‘“‘The Roman Comagmatic Region.” 
He there states that zircon has not hitherto been recorded authentically 
as occurring in the rocks of this region. We are inclined to ask what 
efforts have been put forth to find it. Have the surface deposits, such 
as river sands, been carefully examined ? 
Experience indicates that an examination of the sands lying in the 
streams which flow over the Italian basalts would almost inevitably 
reveal the presence of zircon and other minerals of petrographical 
importance. In making this statement we merely echo a note emitted 
by Bellevue 107 years ago. It should be unnecessary to point out 
that the mere examination of a few rock slices, or the analysis of one 
gram of a crushed rock, is not a fair way of determining the presence 
or absence of zircon in the rocks of a region involving hundreds of 
cubic miles. Perhaps a few figures will make this point more striking. 
We may safely assert that an amount of zircon equivalent to 0°01 
per cent.! of a basalt would probably be overlooked in a chemical 
analysis of the crushed rock, and appears usually to be missed in rock 
sections. And yet a cubic metre of such a rock would carry about 
300 grams of zircon, assuming the latter to be uniformly distributed, 
which it usually is. In other words, a cubic yard of such a rock 
would carry about $1b. of zircon, from which we infer that a cubic 
mile would carry well over a million tons of this mineral. Hence we 
see that hundreds of millions of tons of zircon may be disseminated 
through the igneous rocks of a petrographic province, although it 
would not be detected by the chemical methods now in vogue.” It 
thus seems reasonable to urge that, if we are to talk of comagmatic 
regions, we cannot remain satisfied with the results of analysis applied 
to such small quantities of crushed rock as will barely cover the 
bottom of a crucible. 
Bellevue and Cordier exhibit for us a way out of these difficulties. 
The stream deposits of a region retain the heavy residues which have 
resulted from disintegration carried out through immense periods of time 
and on numberless tons of material. The petrographer has foolishly 
left the study of these deposits to the prospector. From such 
deposits a miner’s panning operation clears out most of the quartz; 
a magnetic separation eliminates the ferromagnetic minerals, and when 
this is done it is very seldom indeed, if ever, that zircon is not left 
behind in considerable quantity, and is readily identified with the 
microscope. If the rocks themselves are to be examined, they may 
be crushed, and in imitation of Chrustschoff, a few hundred grams of 
the crushings may be manipulated by physical methods to secure 
a zircon residue. 
Zircon is not alone in this respect as a mineral the importance of 
whose regional dissemination is liable to be overlooked, though it is 
perhaps the commonest instance. The extensive distribution of 
1 Allowing for the inclusions of such zircons, this would correspond to not more 
than 0-005 per cent. of Zr Oz. 
2 For the estimation of zirconia, Dr. Washington recommends the use of one 
gram of the crushed rock. See ‘‘ Manual of the Chemical Analysis of Rocks,”’ 1904. 
