310 



PEOF. T. Gr. BONNEY ON THE 



altered biotite *, which mineral in an unaltered condition may be 



occasionally detected. 



Just within the embouchure a very singular rock occurs, associated 

 with the ordinary gneiss, and overlying a band of amphibolite. It 

 is very fissile, and appears to consist of flattened lumps, seemingly 

 of white felspar, spotted with quartz, in a grey schistose micaceous 

 matrix. The junction with the overlying gneiss is slightly irregular, 

 and the bed seems rather variable in thickness. On examination 

 with the microscope I find its relations with the gneiss to be closer 

 than I had anticipated in the field. The matrix consists of a mica 

 similar to that in the gneiss, alternating with thin bands of quartz. 

 Both have evidently undergone much disturbance. Some quartz, 

 which is minute and chalcedonic, is certainly of secondary origin ; 

 but the larger grains appear to belong, like the larger mica flakes, 

 to an earlier stage in the history of the rock, as though it had been 

 broken up and recemented. The "lumps" exhibit the same com- 

 pound structure as has been already described in the felspar of the 

 gneiss, but the quartz inclusions are more numerous and the felspar 

 is less characteristic. In one grain these lines of inclusions appear 

 to be bent into a series of arches, the points of which lie in a line 

 roughly parallel with the foliation. Another grain appears to have 

 been broken up and recemented. 



These rocks, then, afford unequivocal indications of mechanical 

 disturbance, undergone after they had assumed a crystalline con- 

 dition, and followed by some amount of mineral rearrangement. 

 There are also marked indications of mineral rearrangement on a 

 larger scale, and it is an interesting question how far that is anterior 

 or posterior to the crushing. As the bands of mica and, to some ex- 

 tent, the inclusions of quartz are parallel with the lines of cleavage, it 

 might be urged that these also had indirectly resulted from the same 

 cause. But to my eye the bulk of the mica has every appearance of 

 being an original constituent. It is sometimes twisted, pushed, and 

 tumbled about, or the ends of the flakes have a " pinched-up " look ; 

 the lines of fracture often cut completely through the felspar-crys- 

 tals with their quartz-inclusions t, and the fragments are occasion- 

 ally separated and displaced (PI. XVII. fig. 2). Hence, though 

 I should admit that a certain amount of mineral deposition and re- 

 crystallization, especially in regard to the quartz, has occurred since 

 the crushing, I believe that when this happened the rock was already 

 foliated. Indeed, apart from microscopic examination, I could not 

 otherwise explain the discordance of the mineral banding and the 

 cleavage already described. "W e have, then, here another case of a 

 cleavage-foliation impressed upon a rock already foliated. What is 

 the history of this earlier foliation our present knowledge does not 



* The " bleaching " being due to the concentration of iron oxide in 1 he cleavage- 

 planes. Cf. the changes which have been described in kypersthene and olivine. 

 Hence the mica has been changed from a magnesia-potash-iron mica to a 

 magnesia-potash mica (perhaps hydrous). 



f The uniform tint of these inclusions suggests that they are secondary and 

 what is often called quartz de corrosion ; but, as stated, I cannot regard these as 

 posterior to the crushing. 



