IN THE NORTH-WEST HIGHLANDS OF SCOTLAND. 391 



S.W. Under the microscope it is found to contain microcline, black 

 mica, calcite, and garnet. It deserves special notice because it is 

 believed to represent the unfoliated form of certain thin veins of 

 mica-schist in the Archaean gneiss, to be afterwards referred to. 



(d) Equally interesting are the intrusive dykes and sheets of 

 granite (or syenite), containing quartz, felspar, mica, and frequently 

 hornblende, traceable from Loch Stack westwards to Loch Laxford. 

 Their general trend is W.jS".W. and E.S.E., and they often coalesce, 

 occasionally forming belts upwards of 500 yards across. Along 

 their lines of outcrop they give rise to conspicuous " slacks " or 

 hollows. From the evidence already obtained in the field, there 

 can be little doubt that they were intruded into the older gneisses 

 after the eruption of the dolerite-dykes ; but whether the injection 

 was prior to the foliation of the dolerite-dykes is not quite so 

 certain. 



5. Later Pre-Camhrian Movements and their Direction. 



We have now reached an important stage in the history of 

 the Archaean rocks in the west of Sutherland ; for after the eruption 

 of these various igneous materials the whole area was subjected to 

 enormous mechanical movements, which exercised a powerful 

 influence both on the dykes and on the crystalline rocks which the 

 dykes traverse. 



These lines of movement run in certain definite directions and 

 may be described as thrust-planes, crush-lines, or lines of shearing, 

 resulting in a newer foliation. They may be grouped in three 

 systems ; — (1) those running more or less parallel with the dykes of 

 basalt-rock, viz. W.N.W. and E.S.E. or N.W. and S.E. ; (2) those 

 trending nearly E. and W. at an oblique angle to the basic dykes ; 

 and (3) those running N.E. and S.W. or N. and S. Of these 

 systems the first and second are by far the most important. Eor 

 the sake of convenience of description it will be desirable to 

 indicate the effects of these movements first on the dykes, and 

 secondly on the gneiss. 



6. Effects of these Movements on the intrusive Dylces. 



When the lines of movement are more or less parallel with 

 the direction of the basic dykes, the dolerites gradually merge into 

 diorites without the development of foliation. Under the influence 

 of enormous pressure the dolerites have undergone complete mole- 

 cular reconstruction ; the felspars become opaque white and the 

 augite is replaced by hornblende, recognizable in the field by its 

 cleavage-angle. Indeed, this molecular change has been so ex- 

 tensively developed in these basic dykes that much of the existing 

 rock deserves the name of diorite. But this type of metamorphosis 

 is only a stage in the conversion of the rock into hornblende-schist. 



When lines of movement coincide with the margins of one of the 

 dolerite-dykes, it usually happens that portions of the outer parts — 

 it may be a few inches or a few feet — are converted into hornblende- 



