174 
IDDINGS. 
of these' kinds of rock. Minette varies from 41 to 67 per 
cent., and kersantite from 45 to 63 per cent., which corre¬ 
sponds to a range of silica from ultra-basic basalt to dacite, 
or nearly the limits of rock variation. The great majority 
of them, however, are below 60 per cent. While very little 
value can be attached to most of these analyses, still some 
general idea of the composition of the rocks can be obtained. 
A ntimber of analyses of minettes have been copied into 
Table VIII, and another group of those of camptonites, 
monchiquites,* and kersantites have been arranged in Table 
IX. With them have been placed two analyses of allied 
rocks, which have been described by Dr. J. Francis Wil¬ 
liams,f whose early death has proved a serious loss to the 
ranks of American geologists. 
The great irregularities that exist in these series of analy¬ 
ses are partly due to the altered condition of the rocks, 
partly to the wide range of rock varieties which have been 
classified under the heads of lamprophyres, and partly to 
inexact methods of analysis. With all these sources of error, 
it is nevertheless apparent that as a whole the minettes are 
characterized by a low percentage of alumina and high 
magnesia, and that the alkalies are relatively high, with 
potash generally in excess of the soda. The analyses of 
kersantite are scarcely distinguishable from those of minette. 
They are characterized by a low percentage of alumina and 
high magnesia. The alkalies are not quite as high as in 
the minettes, and the potash is usually greater than the soda? 
hut several of the analyses in both groups are alike. The 
few complete analyses of camptonites, the typical rocks 
having been described by Hawes, show it to be an extremely 
basic rock, moderately low in alumina, high in ferrous iron, 
and moderately high in lime and magnesia. The alkalies 
*Rosenbusch (H.) and Hunter (M.) TJeber Monchiquite, ein camp- 
toiiitisches Ganggesteine aus der Gefolgschaft der Elaolithsyenite. Min. 
u. petr. Mitth. 8°. Vienna, 1890, vol. 11, p. 446. 
f Williams (J. F.) Annual Report of the Geological Survey of Arkansas 
for 1890. 8°. Little Rock. Vol. 2, p. 107. The Igneous Rocks of Ar¬ 
kansas. 
