254 H. P. CUSHING—SYENITE-PORPHYRY DIKES. 
ferentiation, was much more acidic than was that of Montana. The differ- 
ence between III and LV consists simply in the higher lime and magnesia 
and lower alkalies of the Montana rock, producing pyroxene instead of 
biotite and andesite rather than albite. In I and II these differences 
have almost disappeared, yet are still sufficiently marked to have pro- 
duced a little pyroxene in the one rock and a little biotite in the other. 
The magma from which these rocks were produced belongs to the 
foyaitic type of Rosenbusch. At the one end the silica percentage is so 
high that the boundary between granites and syenites is reached, and 
the rocks might be called either alkali-granite or syenite-porphyries. 
The larger portion of the dikes are quite typical alkali-syenite-porphyries, 
nordmarkite and pulaskite types, the most basic being dikes 32 and 95, 
with 59.2 and 62 per cent of silica.* Between these and dike 9 is a wide 
gap, which it is probable will be filled by future discoveries. Dike 9 is 
rather basic to be classed as nordmarkite-porphyry. It recalls the min- 
ettes, with which it can not be united, owing to its deficient lime and 
magnesia and high alkali percentage. Its deep seated form would be a 
basic alkali-syenite, a rock not yet described, so far as the writer is aware. 
Brogger has described dikes of mica-syenite-porphyry from southern Nor- 
way, but unfortunately just now the writer has not access to his paper 
and does not know whether this basic phase appears or not. Rosenbusch 
mentions an occurrence from the Laurvik district,f characterized by con- 
siderable biotite and apatite and free from quartz, which may be close 
to this rock. We have here a basic alkali-syenite-porphyry, so basic as 
to make it very questionable whether it should be classed with the 
syenites at all. 
GENERAL CONCLUSIONS 
Kemp has shown that along lake Champlain acidic and basic dikes 
occur, which are younger than the Utica slate. Similar dikes occur 
about Montreal associated with nepheline-syenite, all younger than the 
Trenton limestone. At the recent Montreal meeting of the Geological 
Society of America Dr F. D. Adams stated evidence for their late Silu- 
rian and early Devonian age. Similar rocks are reported from many 
points in New England, associated sometimes with nepheline-syenite, 
but owing to the confused stratigraphy often without adequate determi- 
nation of their age. To the westward in Canada, so far as the writer is 
aware, they are either lacking or have not yet been differentiated from 
the older eruptives.{ 
* Determined by E. W. Morley. 
+ Microskopische Physiographie der Massigen Gesteine, p. 469. 
{ Conf. N. H. Winchell on ‘“‘ The latest eruptives of the Lake Superior region,” Am. Geol., vol. xvi, 
pp. 269-274. 
