444 Prof. Dr. T. G. Bonney — TroldoUte, etc., in Aberdeenshire. 



intruded into another, but the boundary is irregular, the felspathic 

 portion of the troktolite seeming to pierce or send off-shoots into 

 the serpentine. This debateable region may be about one-eighth of 

 an inch wide. As in addition each rock in the slide is about of its 

 normal coarseness, I should conjecture that the intrusion took place 

 when the older rock was at a high temperature, and perhaps 

 was still plastic. Which, then, was the earlier? At Coverack, in 

 the instances on the West Coast of Scotland described by Prof. 

 Judd, and in several cases where I have examined the junctions of 

 ordinary gabbros and peridotic serpentines, the felspathic rock is the 

 later of the two; though I may add that its coarse crystallization 

 seems to indicate that its cooling was very gradual. In this case it is 

 more difficult to come to a conclusion. The felspar of the troktolite 

 near to the junction appears to occur in slightly smaller grains than 

 in the normal rock, but the difference is slight and the rock variable. 

 There is, however, one peculiarity. Near to the junction every grain 

 of olivine is surrounded by a fringe-like border, resembling an 

 incipient spherulitic structure. In some cases this appears double, 

 the zone nearest the olivine consisting possibly of a serpentinous 

 mineral ; but the outer, and in some cases the only mineral occurring, 

 is a long and somewhat fibrous microlith, which from its general 

 aspect and extinction-angle I should judge to be actinolite or trerao- 

 lite.^ In these slides the flakes or border of diallage noticed in the 

 normal rock are wanting. It is possible that the structure might be 

 a result of the intrusion of the serpentine into the troktolite ; but on 

 the whole I incline to the opinion — though not without hesitation — 

 that the former rock is the earlier. 



The Black Dog is a mass of rock about four yards long and 

 broad, and rather more than two high, which projects from the sand 

 and bears a very rude resemblance to the creature from which it is 

 named. A dozen or more of much smaller blocks ai'e scattered 

 about in the neighbourhood, some a little nearer to, some a little 

 further from the land. Most, if not all, of these can be visited at 

 low water. Rain and wind prevented me from attempting to plot 

 down these blocks (whether they are in situ or boulders I cannot 

 say), and obliged me to content myself with a rather hasty examin- 

 ation of them. Professor Heddle speaks of two masses of serpentine, 

 a little seaward of the Black Dog. I obtained my specimens from 

 two masses rather on the landward side (and to north of it), and 

 there was a third not far from these, but I noticed two or three 

 more dark blocks surrounded by the sea, which may have been 

 those spoken of by Professor Heddle. Of the remaining blocks, one 

 was certainly a dark gneiss ; the others appeared to me to resemble 

 the rock of the Black Dog, but at these I barely glanced.^ Macro- 



^ These or other microliths sometimes pierce the felspathic part of the slide in a 

 way that recalls the canal system of Eozoon, but to discuss this would require too 

 long a digression for the present occasion. 



^ Boulders are not common on this part of the shore. In fact, I do not remember 

 observing one for about a mile and a half to the south, so that the rock probably is 

 either in situ or has come from the immediate neighbourhood. 



