184 
character of true rocks. It might, therefore, be natural to give 
to each of the three kinds of sheets a distinct name of its 
own. The reasons why it has here been preferred to introduce 
one term comprising all the sheets are the following: — (1) The 
entire series of about one hundred white, red, and black sheets 
constitutes a rock-body which must, for geological reasons, be 
regarded as a unit and needs a name of its own. (2) There 
are no indications whatever that three kinds of magma corre- 
sponding with the three kinds of kakortokite-sheets have existed 
at any time. As it will be shown in the last chapter it must 
be assumed that there has been only one kakortokitic magma 
which was differentiated in place during the process of crystal- 
lization. (3) As previously mentioned the sheeted kakortokite- 
body passes into an unsheeted kakortokite both at its upper 
boundary and at the northeastern end, and this undifferentiated 
kakortokite is of a mineral composition which is intermediate 
between the three kinds of sheets. 
In A of the above table (p. 182) is given the supposed 
mean composition of the kakortokite. In order to obtain this 
the three analyses of kakortokites have been calculated to 100 
per cent. omitting H,O, Е, and Nb,O, (see table at the end 
of the summary). From the figures obtained in this way the 
mean composition has been calculated under the assumption 
that the total thickness of the white sheets is three times as 
great as the total thickness of the black sheets, and nine times 
as great as that of the red ones. This assumption, however, 
is somewhat arbitrary since the observations relating to the 
thicknesses of the sheets are very incomplete (see р. 46). 
