GABBROS OF EAST SOOKE AND ROCKY POINT, 17 
gram figured, which has been contrived by Mead, is so divided 
that a finite line represents any quantity up to infinity. One of 
the horizontal lines is allotted to each of the component oxides 
of the rock. The position of the point on each line, which repre- 
sents the change of that oxide during alteration, is obtained by 
dividing the percentage of that component in the chemical 
analysis of the fresh rock by its percentage in the altered rock, 
and multiplying the result by 100. The points so obtained on 
the various horizontals may then be connected by lines. The 
result shows at a glance the relative gains and losses during 
alteration, or the absolute gains and losses if any factor is known 
to have remained constant. Thus, if any constituent has 
remained constant, then all of the constituents whose points fall 
to the right of the known point have decreased in absolute 
amount, and the constituents whose points fall to the left have 
increased. If weight has remained constant, i.e., if 100 grams of 
fresh has yielded 100 grams of altered rock, then all consti- 
tuents whose points lie to the right of the vertical 100-line have 
decreased in absolute amount, those whose points lie to the left 
of this line have increased. If an absolute change in weight can 
be determined, i.e., if 100 grams of fresh are known to have 
yielded 90 grams of altered rock, then the vertical line 90 is the 
zero line, and points to the right or left of this represent abso- 
lute losses or gains respectively. When lack of information 
renders it impossible to fix any point as constant, the relative 
gains and losses of the different constituents are all that can be 
determined. 
66&73 — A 
