THE SUDBURY LACCOLITHIC SHEET 779 
miles, so that they are insignificant in amount as compared with the 
nickel-bearing sheet. 
Other eruptives—There is another mass of eruptive materials, 
the Onaping tuff, consisting of volcanic ash and lapilli, which may be 
considered as probably having its source in the original magma; 
though now spread out as a thick layer between the ordinary water- 
formed sediments overlying the nickel eruptive. As its area is at 
least 200 square miles and its thickness two-thirds of a mile, its vol- 
ume must be about 130 cubic miles; and before it underwent erosion 
it must have been far more extensive than now. Much of its materials 
fell as angular fragments of glass, now transformed into chalcedony, 
serpentine, etc., but mingled with the pyroclastic matter there are a 
good many well-rounded pebbles and bowlders of quartzite and 
granite, suggesting ordinary water action. Probably the ash fell into 
the same body of water which deposited the other sediments of the 
basin. 
An analysis of the tuff by Professor Walker gives the following 
results: 
SLO) Fo earO en nornt cores anh mee ta edNee Bey S O03 
TULA eae ae oa amen emg rt kale tree aA” Bums Bejati i) 
COU eh ese Ne) a ea ens ea ee ee ONS O 
IM Oe ie unrest yhiah eatery hat, Boise ntemele ie pounds AS eT O) 
CAO TON oe mei ee iter nner jie SAGA 
INAS OR) eects as iehe® sii tara Fant alee a eee tt OO 
KG OS ong at eee aoe ee an ature .97 
Min © pate i Rt eee Beaten: fa Par ardeaoonn aieags witha Ce 
oss sbysiomitiony \iarseee ieran wu lernel ale clears rele 
Motala 0 Vase 5 cee oe en he SEO SROs 
Its composition is somewhat different from the average of the 
nickel eruptive as given on a former page, but not so different that we 
cannot suppose it to have come from that source before the laccolithic 
sheet had reached its present position. The vitrophyre tuff must 
have been flung from some volcano of which we no longer have a 
trace, at a distinctly later time than the lava sheets of basic norite 
described before, since we have reason to believe that the whole 
series of sediments of which the tuff forms a part extended over the 
area occupied by the older norite before the sheet of nickel eruptive 
was injected between them. 
