222 Dr. C. Callaway — Notes on Metamorphism. 



gneiss appears to conformably overlie the quartzite, the whole 

 dipping easterly. The bed of quartzite in contact with the overlying 

 gneiss looks very much like a quartzose gneiss, and would no doubt 

 be so regarded by many geologists. But, in addition to the evidence 

 furnished by the section, the microscope affords valuable testimony to 

 the true nature of the rock. A specimen from this locality is grouped 

 •with one from the base of the Stack of Glen Coul in the following 

 description : — " They may be called quartzites ; for the mineral is 

 chiefly quartz, but it is extremely difficult, especially in the former 

 (from Glen Coul), to detect with certainty the original fragments. 

 Viewed with crossed Nicols, the slide appears to be composed of 

 minute granules of quartz of chalcedonic aspect ; among these are 

 vi^avy, somewhat parallel bands, which appear almost homogeneous, 

 but break up like the rest as the stage is rotated, though occasionally 

 an irregular nucleus appears to remain homogeneous. These give 

 a streaky or somewhat foliated aspect to the slide. There is a little 

 sericite, which enhances the structure. In the Glen Coul specimen, 

 however, which contains a little hsematite, some few original grains of 

 quartz may be recognized, together with a little microline. I should 

 suppose these rocks to be exceptional varieties of the quartzite 

 group : but the obliteration of the characteristic structure, and the 

 resemblance to the quartzose part of many highly-altered mica- 

 schists, suggests that the rock may have been originally rather a fine- 

 grained sand or silt, and then, by the action of heat, pressure, and 

 water, almost reduced to a gelatinous silica ; so that, as in a schist, 

 many of the minute grains of quartz now visible are of secondary 

 origin." 



We thus see that a gradation may be traced between ordinary 

 quartzite and a rock which has many of the characters of a true 

 schist ; but in which, under the microscope, slight traces of the frag- 

 mental structure may still be detected. A Murchisonian would, 

 therefore, be almost certain to quote such examples as these in support 

 of the theory to which his leader, by the sheer force of authority, 

 compelled the assent of the scientific world. 



While examining these interesting rocks, I became convinced that 

 pressure has had more to do with metamorphism than has commonly 

 been supposed. On Loch Glen Coul, where masses of Hebridean 

 gneiss rest upon the Assynt group, I could detect no material altera- 

 tion in the latter ; and here accordingly there is no evidence of 

 extraordinary pressure. Up the glen, however, where the progressive 

 alteration is seen, we are in the focus of an enormous squeeze. The 

 quartzite is reflexed again and again in closely adpressed folds, as if 

 it were too tightly folded in with the Hebridean to suffer fracture. 

 But on the loch, the quartzite and associated beds are shivered into 

 a chaos of fragments, as if, being unsupported from behind, they had 

 relieved the pressure by giving way. Here, they are not folded 

 with the Hebridean : but great masses of the gneiss have been 

 thrown over on to the top of them. The mere. weight of these over- 

 lying beds must have exerted a pressure incomparably less than 

 the great earth-thrust which has so intensely squeezed and contorted 



