186 '3RANITES AND GRANITIC VEIN-STONES. [XI. 



deposition, or from the flowing of a semi-fluid heterogeneous 

 mass giving rise to a stratiform arrangement.* 



4. The rocks having the mineralogical composition of 

 granites present a gradual passage from the coarse structure of 



[* This process has been particularly described in my Contributions to 

 Lithology, where also the principles of lithological classification are discussed 

 at length. (American Journal of Science for March and July, 1864. ) A strati- 

 form structure in eruptive rocks is there said to be clue to " the arrangement 

 of crystals during the movement of the half-liquid crystalline mass, but it may 

 in some instances arise from the subsequent formation of crystals, arranged 

 in parallel planes." In the same paper, in describing the dolerite of Montar- 

 ville, the alternations of a coarse variety, porphyritic from the presence of 

 large crystals of augite, with a finer grained and whiter variety is noticed; 

 the two being " arranged in bands, whose varying thickness and curving lines 

 suggest the notion that they have been produced by the flow and the partial 

 commingling of two fluid masses." At Mount Royal also, as there described, 

 " mixtures of axigite with feldspar are met with, constituting a granitoid 

 dolerite, in parts of which the feldspar predominates, giving rise to a light 

 grayish rock. Portions of this are sometimes found limited on either side by 

 bands of nearly pure black pyroxenite, giving at first sight an aspect of strati- 

 fication. The bands of these two varieties are found curiously contorted and 

 interrupted, and, as at Montarville, seem to have resulted from movements in 

 a heterogeneous pasty mass, which have effected a partial blending of an augitic 

 magma with another more feldspathic in its nature." 



Further illustrations of this are given by the author in a communication 

 to the Boston Society of Natural History, January 7, 1874. Among these 

 was a specimen from Groton, Connecticut, in which a large angular fragment 

 of strongly banded micaceous gneiss is ^closed in a fine-grained eruptive 

 granite, the mica plates in which are so arranged as to show a beautiful and 

 even stratification in contact with the broken edges of the gneiss, but at right 

 angles to the strata of the latter. Another example is afforded by the erup- 

 tive diorite from the mesozoic sandstone of Lambertville, New Jersey, which 

 is conspicuously marked by light and dark bands due to the alternate pre- 

 dominance of one or the other of the constituent minerals ; and still another 

 in a fine-grained dark micaceous dolerite dike from the Trenton limestone at 

 Montreal, in which the abundant laminae of mica (probably biotite) are ar- 

 ranged parallel to the walls of the dike. A similar banded structure is seen 

 in glacier-ice and in furnace-slags. Some geologists have from facts of this 

 kind been led to suppose that the banded structure of great areas of gneiss \v;is 

 caused by movements of flow in a solidifying mass, and not by successive de- 

 posits of dissolved or suspended material from a watery medium. While ad- 

 mitting the frequent occurrence of this structure in eruptive rocks, and the 

 necessity in many cases of a careful geognostical study to determine to which 

 clan a stratiform rock should be referred, it was maintained that the great 

 areas of gneissic rocks are of aqueous origin, and were deposited in successive 

 horizontal layers with their associated limestones, quartzites, and iron-oxides.] 



