TheTlan of the Earth and its Causes. — Gregory. 123 
Igneous Rock-Series and Mixed Rocks. By Alfred Marker, (four. 
Geol., 8, 389-399) 
A rock-series is defined as an assemblage of rock-types, with a cer- 
tain community of characters, associated in the same district, belonging 
to the same suite of eruptions and holding a similar position in the 
scheme of igneous rocks belonging to that suite. According to the dif- 
ferentiation hypothesis, they are derivatives of the same order from one 
common source, resulting from differentiation along similar lines and 
tc the same degree. The fundamental characteristics of such a series, 
having regard to chemical composition, are of two kinds: (i) those 
belonging to the individual rock-types and shared by all the types in- 
cluded in the series; and (2) those belonging to the assemblage of 
types as a whole, depending upon variations in the composition of the 
members as compared with one another. These characteristics admit 
of very clear graphic presentation. For this purpose the method of 
two equal rectangular co-ordinates first used by Iddings is recom- 
mended, silica being referred to one axis and bases to the other; and 
the resulting curves are briefly discussed. The origin of ingeous 
rocks by admixture rather than by differentiation is next considered, 
and three principal cases are distinguished: (i) mixture of two fluid 
magmas; (2) permeation of a solid rock by a fluid magma; and (3) 
inclusion of rock fragments in a fluid magma. The second and third 
cases are practically limited to igneous rocks of cognate origin ; and 
the distinction of the included and permeated rocks or xenoliths as 
accidental and cognate is regarded as important ; the latter only yield- 
ing by their absorption new rocks of any considerable extent or im- 
portance. Several cases are considered, including the admixture of 
two rocks of the same rock-series, and the solution by a magma of 
extraneous quartz, and of limestone. The admixture of the extreme 
types of a series will not, in general, give exactly any of the inter- 
mediate types. The rock-analyses of Clarke and Hillebrand are cited 
in illustration of this principle ; and it is pointed out that mixtures, even 
of two normal igneous rocks and still more of an igneous and a sedi- 
mentary rock, must often be abnormal in chemical composition. The 
relations of the chemical composition of the magma and of foreign 
admixtures to the mineralogical compostion are also considered. 
The Sundal Drainage System in Central Norway. By R. L. Barrett. 
(Bulletin American Geographical Society. No. 3, 1900.) 
The young Sundal drainage system, on the northern coast of cen- 
tral Norway, possesses deep gorges and canyons which drain north- 
west into the Atlantic. It is closely related to, but divided from, the 
comparatively shallower and more or less disconnected valley system 
of the Opdal, which drains northeast but also reaches the Atlantic. 
The divide between these two systems has been shifted from east 
to west, as has been the case in a less degree in the two drainage sys- 
tems just south of the Sundal, the Eikedal and the Ronesdal. 
The paper presents facts to prove that the divide in question once 
stood near the head of the Sundal fjord, more than sixty-five kilo- 
