Hunt.] 238 [January 7, 



Of a different origin is the stratiform structure often seen in rocks 

 clearly exotic or erupted, which is apparently due to the arrangement 

 of the elements in a flowing and imperfectly liquid material. This 

 was well shown in a specimen from Groton, Connecticut, in which a 

 large angular fragment of strongly banded micaceous gneiss is in- 

 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. A coarse-grained dolerite, from Montreal, Canada, 

 was also shown, in which black augite crystals in bands of half an 

 inch in width alternate with others of nearly unmixed white labra- 

 dorite. These bands, which may be traced for a distance of several 

 feet on glaciated surfaces, are found curiously contorted and inter- 

 rupted, and in their drawn-out and lenticular arrangement suggest 

 the extension by flow of a heterogeneous pasty mass, and the partial 

 blending of an augitic portion with another more feldspathic. Simi- 

 lar appearances are equally conspicuous in the dolerite of Montar- 

 ville, a few miles distant from the last. These dolerites were erupted 

 probably before the Devonian period. A fine-grained, dark mica- 

 ceous dolerite from a narrow dike cutting the Trenton limestone near 

 Montreal was also exhibited, in which the abundant laminae of mica 

 (probably biotite) are arranged parallel to the walls of the dike. An 

 eruptive diorite, shown from among the mesozoic sandstones at Lam- 

 bertville, New Jersey, is conspicuously marked by light and dark 

 bands, due to the alternate predominance of one or the other of the 

 constituent minerals. 1 The speaker alluded to these cases of stratifi- 

 cation in eruptive rocks only as fresh illustrations of a well-known 

 phenomenon which has been repeatedly observed and described by 

 geologists. A similar stratified structure is also seen in glacier-ice 

 and in many furnace-slags. The consideration of such facts has led 

 some geologists to suppose that the banded structure of the great 

 areas of gneiss and gneissoid rocks was caused by movements of flow 

 in a solidifying mass, and not to successive deposits of dissolved or 

 suspended material from a watery medium. While admitting the 

 frequent occurrence of this structure in eruptive rocks, and the neces- 



1 From the study of the diorites interstratified with the mesozoic sandstones of 

 New Jersey, Prof. Henry "Wurtz has been led to look upon them as indigenous 

 rocks, like the diorites of the Huronian series of crystalline schists, which they in 

 many respects resemble. Notwithstanding his great respect for the opinions of 

 this learned naturalist, Dr. Hunt, in common with most other geologists, regarded 

 these mesozoic diorites as intrusive. 



