ASSIMILATION OF THE SYENITE-GRANITE MAGMA 255 



with igneous material and on the other an igneous rock which has melted into 

 itself or assimilated sedimentary material. Between these two types every 

 gradation exists." ^^ 



'No details, however, are given. 



Observations of H. P. Cushing. — Gushing states that the differentia- 

 tion of the basic border phase of the Tupper syenite of the Long Lake 

 quadrangle 



"seems conditioned on the nature of the bordering rock. The most of the basic 

 syenite and all of the more gabbroic of it is in close association with the 

 anorthosite gabbro border." ^" 



"These relations are readily explained on the assumption that there has been 

 actual incorporation and digestion of material from the surrounding rocks by 

 the syenite, and that the digestion has been mainly a border phenomenon. 

 Nothing that was observed is antagonistic to such an explanation. No other 

 solution that will account for the observed relations suggest itself." ^^ 



Still more recently, Cushing has clearly recognized the influence of 

 magmatic assimilation in the Thousand Islands region. He says : 



"The granite dikes usually represent the extreme acid state of the rock 

 (Laurentian granite). The main mass averages less acid, chiefly because of 

 the inclusions (amphibolite) and the attack of the granite on them. In its 

 preliminary stages this usually takes the form of an injection of the granite 

 in thin sheets along the foliation planes of the amphibolite, the so-called 'lit- 

 par-lit,' or leaf type of injection. . . . Then, here and there the granite 

 breaks out from the foliation planes and spreads through the rock adjacent. 

 . . . This process becomes more and more pronounced, until much of the 

 rock is broken up into a granular mosaic of particles cemented together by 

 granite films, producing what may be called a mosaic type of injection. . . . 

 As a further stage in both types of injection the sharp boundaries become 

 blurred, and this shading of the two rocks into one another becomes more and 

 more prominent, until finally rocks result, which seem unquestionably to be 

 due to the complete digestion of the amphibolite by the granite, gray gneisses 

 of intermediate composition." ^^ 4 



Observations of the writer. — A large number of actual examples of 

 magmatic assimilation have come under the writer's observation, but with 

 few exceptions these are all of small areal extent. . The purpose is to 

 describe only a few of the most typical and carefully studied examples, 

 or just enough to make clear the nature of the evidence and the character 

 and extent of the resulting rocks. 



The most frequently occurring cases are those of small Grenville inclu- 



"C. H. Smyth, Jr. : N. Y. State Mus. Bull. 158. 1912, pp. 148-144. 



20 H. P. Cushing: N. Y. State Mus. Bull. 115, 1907, p. 479. 



21 H. P. Cushing : Bull. Geol. Soc. Am., vol. 18, 1907, p. 484. 



22 H. P. Cushing : N. Y. State Mus. Bull. 145, 1910, p. 37. 



