562 
transition is sharp there is no evidence of sepa- 
rate intrusive masses.2 
The interbanding of syenite and granite 
above cited as occurring in the Long Lake and 
Elizabethtown quadrangles are by Cushing 
and Kemp, respectively, interpreted as being 
most likely due to some process of magmatic 
differentiation. Kemp says that one is not 
“justified in inferring more than a difterentia- 
tion of an eruptive mass into layers or portions 
of contrasted composition.” For most cases 
throughout the Adirondacks, especially the 
very common occurrence of banded variations 
like those illustrated in the above examples, 
the writer agrees with this interpretation since 
there appears to be no escape from the idea of 
some sort of differentiation of the magma into 
layers of varying composition. Transitions be- 
tween layers range from sharp to very gradual, 
but in a typical case the whole body of rock is, 
as Cushing says, “manifestly bound together 
as a mass of eruptive material arising from a 
common magma.” Whether or not the transi- 
tions are sharp, they are always marked by 
interlocking crystals. 
But what were the physical conditions under 
which the differentiation occurred? Did the 
differentiation take place before, or after, or 
during the process of intrusion? The writer 
ventures to offer some suggestions by way of 
partial answers to these questions. 
M. E. Wilson has discussed the banded 
gneisses of the Laurentian Highlands of 
Canada* which are essentially very similar to 
those of the Adirondacks. In his summary 
Wilson says: “As regards the origin of the 
folded, banded and foliated structure of the 
eneisses, it is concluded that these are all 
genetically related in the Laurentian moun- 
tain-building deformation which acted upon 
the magmatic axil mass during its consolida- 
tion ” and “that the principal factor in bring- 
ing about the heterogeneity of the Laurentian 
complex was differentiation aided by (oro- 
genic) deformation during its consolidation.” 
Now, in the main portion of the Adirondack 
region the banded syenite-granite series shows 
2N. Y. State Mus. Bull. 138, p. 48. 
8 Am. Jour. Sci., Vol. 36, pp. 109-122. 
SCIENCE 
[N. 8. Von. XLVITI. No. 1249 
little, if any, folding due to orogenic pressure, 
and the foliation is essentially a magmatic 
flow-structure produced under moderate pres- 
sure, that is a pressure little or no greater than 
that which resulted from the shouldering 
action of the syenite-granite magma during 
its intrusion. Reasons for these conclusions 
are given at some length by the writer in a 
recent paper. Such being the case, orogenic 
pressure was not a principal factor in the pro- 
feature of the whole central belt of Laurentian 
duction of the banded structures of the 
Adirondack syenite-granite series. Wilson 
states that the banding is a very persistent 
Highland gneisses, but in the Adirondacks the 
more localized developments of pronounced 
banded structures strongly oppose the idea 
that they were produced under general re- 
gional or orogenic pressure. The writer be- 
lieves, therefore, that orogenic pressure has 
not been a necessary condition for the pro- 
duction of banded gneisses such as those de- 
scribed in this paper. 
It may well be conceived, however, that, in 
those portions of the rising magma. where the 
shouldering pressure was greater, the differen- 
tiation into layers of contrasting composition, 
color, texture and foliation proceeded more 
readily, while in other (often large) portions 
of the magma, where the shouldering pressure 
was relatively slight, the conditions for differ- 
entiation into contrasting bands were not so 
favorable. The influence of pressure in the 
production of the banding is thus cecognized. 
It is further believed that the syenite-granite 
magma rose very slowly and irregularly, and 
that there was differential magmatic flowage, 
especially in those portions where the con- 
trasting bands were developing. Many of the 
bands are not considered to have consolidated 
simultaneously sinte alternating bands show- 
ing sharp differences in degree of magmatic 
flow-structure foliation prove that some of 
the layers were more fluid and continued to 
flow after adjacent layers were wholly or 
nearly consolidated. Accordingly, where the 
banded structures are well developed we may 
picture not only the slow intrusion of the 
4 Jour. Geol., Vol. 24, pp. 587-619. 
