444 D. Balsillie — 



endowed with the qualities that experience has testified as best 

 suited to the requirements of the engineer. Thus the rock of 

 Kilbrackmont Craig, than which there is no fresher or more compact 

 dolerite in East Fife, is not esteemed a first-class road-stone, and 

 this is due to its smoothness of fracture. That appears to be 

 a serious objection to its use, for it has been found that a road 

 constructed with, this metal suffers especially from the plucking 

 action or suction of motor tyres. The units composing the surface 

 rapidly become dislodged, and this can be attributed to a lack of 

 frictional binding. Texture, therefore, must be regarded as 

 a lithologic character of particular significance to the road surveyor. 

 But in assessing the value of any rock as a medium for the repair 

 and maintenance of roads, there are other factors bearing upon the 

 matter to which also serious consideration must be given. One 

 of these is the quality of brittleness, for it has frequently been 

 observed that the same stone may endure differently, depending 

 on the nature of the sole or foundation upon which it is superposed, 

 and that what under certain conditions would furnish excellent 

 setts or macadam, might be less suitable for the like purpose if 

 departure be made therefrom. 



Petrographically, the olivine-bearing dolerites of East Fife may 

 be allocated to three principal types, viz. : — 



(1) The Gathercauld type — an ophitic type. 



(2) The Kilbrackmont and Baldutho type — a porphyritic type. 



(3) The Kingask-Spalefield type — a picrite-teschenite type. 



(1) The Gathercauld type. — The dolerite that furnishes material 

 for a description of this type forms the large intrusion stretching 

 south from Baldinnie to Gathercauld and terminating westwards 

 in a finely columnar escarpment that has been noted by Sir Archibald 

 Geikie. In hand specimen the rock is dark coloured, basic looking, 

 fairly coarse-textured, and showing sometimes large crystals of 

 pale yellow olivine. Microscopically, the mass consists of a frame- 

 work or plexus of plagioclase laths, held together by ophitically 

 related clumps of monoclinic pyroxene. In the interstices occurs 

 another and later felspar, while throughout the whole and generally 

 antecedent to the other minerals in an assumption of the solid 

 phase are distributed olivine and accessory constitaents. Along 

 with the later felspar residual glass and analcdte occur in small 

 amount. 



The felspar of the Gathercauld dolerite does not appear to be 

 possessed of high refractive indices in any part of the rock. By 

 optical means it can be determined that there is a range of plagio- 

 clase from labradorite, with from 60-65% of anorthite, through 

 andesine to oligoclase and oligoclase-albite. (Note. — Very basic 

 felspar does not seem to occur in any of the rocks under 

 discussion ; thus there is a quartz-dolerite exposed in a disused 

 quarry at Lade Braes, St. Andrew's, the felspar of which is more 

 anorthitic than in the case of any of the olivine-dolerites.) A brief 



