ADIRONDACK BASIC INTRUSIVES 179 
gravity figures given by Daly" on good authority it seems quite 
certain that most, at least, of the xenoliths would have sunk in 
the North Creek stock magmas during their highly molten early 
stages of intrusion. 
The very close chemical relationship between the diabase and 
the typical gabbro pretty clearly shows the derivation of these 
rocks from a source of supply of such a basic composition, and that 
the acidic facies of the gabbro were somehow developed during 
the process of intrusion, more than likely by the assimilation of 
country rock. Where the normal basic rock now constitutes a 
stock is readily explained because in the later, less highly molten 
stage of intrusion, stoping would be greatly lessened and xenoliths 
would sink little if any, while this upward current of later basic 
magma would push the earlier formed, more acidic ‘‘syntectic”’ 
to a higher level in the vent. Where basic gabbro now fills a stock 
chamber simply means that the upper, more acidic assimilation 
product (syntectic) has been removed by erosion. 
The objection may be raised that these stocks are too small to 
have had sufficient heat for the melting and diffusion of the xeno- 
liths. Butit must be remembered that the gabbro and its derivatives 
are true plutonic rocks, and that the portions of the stocks now at 
the surface were, at the time of intrusion, far below the surface. 
The fact that, in some cases, tongues of the intrusives have pene- 
trated the country rock clearly argues for a highly fluid magma. 
Again, the very character of the distribution of the gabbros along 
distinct belts strongly suggests that the stock masses are merely 
offshoots from a much larger underground gabbro mass which, 
at the time of the intrusion, must have been more highly molten 
than the present surface exposures seem to indicate. 
Thus the North Creek basic stocks appear to furnish fine 
illustrations of intrusions accompanied by more or less magmatic 
stoping and assimilation, so that we have all gradations from basic 
gabbro to syenite stocks, and from stocks in which magmatic 
assimilation produced true syntectic magmas, to those in which 
xenoliths were only partially dissolved and diffused, to those in 
which xenoliths were wholly undissolved. 
t Amer. Jour. Sci., XXVI (1908), 27-28. 
