142 OUTLINES OF FIELD-GEOLOGY PART I 



suffered from great lateral pressure, the dislocations, 

 which are forms of reversed faults, sometimes occur as 

 inclined or undulating planes, the rocks above which 

 have been pushed over those below. The horizontal 

 displacement in such cases sometimes amounts to many 



FIG. 40. -View of north face of Beinn a 1 Mhuinidh, Loch Maree, showing how a 

 cake of Lewisian gneiss, etc., has been pushed along a thrust -plane, and now 

 lies above Cambrian dolomite and quartzite. (A section across this hill- 

 front is given in Fig. 41.) 



miles. Not only have the rocks been ruptured, and older 

 deep-seated masses been torn up and driven bodily over 

 younger formations, but there has been at the same time 

 such an amount of internal shearing as to crush the 

 rocks into a finely-divided material (mylonite), and to 

 give rise to a streaky arrangement of the broken particles, 

 closely resembling the flow-structure of a lava. Coarse 

 pegmatites may be traced until they pass into a 

 substance that might easily be mistaken for an ancient 

 rhyolite. In the crushed material new minerals have 

 been sometimes so developed as to produce a true 

 schist. 



A thrust -plane may in places be almost horizontal, 



