20 J. G. GoodcJiild — Aug en-structure and Eruptive Rocks, etc. 



in Dinornis. The intercondylar fossa is scarcely perceptible and the 

 rotular surface is little depressed ; owing to fracture it cannot be seen 

 how far the condyles projected forwards beyond its level. 



In the same collection is a very incomplete proximal end of a 

 metatarsus, perhaps belonging to this species. 



IV. — Augen-Stkuoture in Eelation to the Origin of the 



Eruptive Eooks and Gneiss.' 



By J. 6. GooDCHiLD, F.G.S., H.M. Geological Survey. 



AMONGST the rocks whose original structures have been more 

 or less deformed by metamorphic agencies the geologist very 

 commonly meets with curious eye-like inclusions of mineral matter, 

 which are generally known by the name of augen. Close investiga- 

 tion of a large series of rocks exhibiting these eyes brings to light 

 the fact that under this name are classed two essentially different kinds 

 of structure. In the one, the kernels or eyes are manifestly the 

 unsheared portions of the rock, whose sheared portions constitute the 

 schist that envelops the eyes. In the other kind of augen structure 

 the eyes consist of crystalline minerals, whose development as such 

 is shewn to be posterior to the shearing to which tlie matrix has 

 been reduced, by the fact that the eyes of crystalline matter have not 

 participated in the shearing, but are bright and generally unfractured 

 throughout. It is here suggested that the term "flaser structure" 

 should be confined to the results of uncompleted shearing, where the 

 eyes were formed contemporaneously with the movements ; while to 

 the structure seen in the rock containing the crystalline augen 

 developed after the shearing movements had ceased, it is suggested 

 that the term " augen structure " should be restricted. 



Augen structure occurs under two sets of conditions. In the one, 

 the constituents out of which the augen have been formed were 

 already in existence within segregating distance, when the rock was 

 first subjected to metamorphic action, and their development is 

 merely a case of regeneration under plutonic conditions, whose 

 general nature will be discussed presently. In the other set of 

 augen, the rock in which the structure occurs did not originally 

 contain the whole of the constituents that were essential for the 

 formation of the eyes. Part of these constituents must, therefore, 

 have been introduced from an outside source, some time late in the 

 history of the rock, but possibly only a short time prior to the date 

 when the causes concerned in the development of the augen came 

 into action. In the simpler case, where the constituents existed 

 within the rock from the first (which may be typified by augen am- 

 phibolite), a crystalline development has been set up in the i-ock at 

 various disconnected points, while the surrounding material, although 

 very generally granulitised, still remains in a schistose state. 



Assuming that heat, co-operating in any one of various ways 

 with chemical action, is an essential factor in the generation of the 



1 This is substantially part of a Lecture on the " Causes of Volcanic Action," 

 delivered under the auspices of the Eoyal Scottish Geographical Society at Edinburgh, 

 December, 1892. 



