254 W. J. MILLER MAGMATIC DIFFERENTIATION AND ASSIMILATION 



bases, which are either less or not at all metamorphosed, and hence show 

 relationships more clearly. Thus, in the North Creek quadrangle, the 

 writer has observed in a gabbro dike I14 miles a little west of south of 

 South Horicon fairly coarse-grained gabbro in sharp contact with fine- 

 grained gabbro, the latter gradually becoming coarser again away from 

 the contact, and clearly showing a second intrusion of rock much like the 

 first after the first had partly or wholly solidified. Also Kemp says, con- 

 cerning a diabase dike of the Elizabethtown quadrangle: 



"One very interesting case has been found of one dike penetrating another 

 and chilled by it. There were clearly two periods of intrusion in this instance, 

 and one followed long enough after the other to have permitted the- first to 

 quite thoroughly cool." " 



Such actual examples of separate intrusions of the same magma 

 strongly support the idea that similar tilings might be expected in connec- 

 tion with the larger syenite-granite magma. 



Twenty years ago Barlow said, concerning certain granite and granite 

 gneisses north of Lake Huron, that these rocks frequently grade into each 

 other, while in some- cases granite cuts the gneiss. After giving detailed 

 reasons, he concluded that : 



"These masses of granite may therefore be regarded as non-foliated areas 

 of gneiss, representing simply certain eruptions from the same fluid magma 

 from which the gneiss itself has solidified, and although a suflicient time had 

 elapsed to allow of more or less complete consolidation of the gneiss, yet they 

 represent the same age in geologic time." " 



Assimilation by the Syexite-Granite Magma 



actual examples 



Observations of C. H. Smyth, Jr. — In his description of Grenville 

 gneiss inclusions, from a few feet to a few rods across, in the western 

 Adirondack region, Smyth says : 



"Some of these inclusions are clearly defined with sharp boundaries, but 

 others are blended with the surrounding syenite as though they had undergone 

 a partial melting." ^^ 



Again, in a recent paper on the pyrite deposits of Saint Lawrence 

 County, Sm}i:h says : 



"It often happens that a rock is of composite character as a result of injec- 

 tion or assimilation, giving, on the one hand, a sediment more or less 'soaked' 



18 J. F. Kemp : N. Y. State Mus. Bull. 138, 1910. p. 58. 



"A. E. Barlow: B\ill. Geol. Soc. Am., vol. 4, 1893. p. 315. 



" C. H. Smyth, Jr. : 18th Ann. Kept. N. Y. State Geologist, 1898, p. 477. 



