Prof. Br. T. G. Bonney — TroMoUte, etc., in Aberdeenshire. 443 



I may call attention to another point of resemblance between the 

 Cornish and Belhelvie rocks in their relation to the serpentine. At 

 Coverack Cove the serpentine is the oldest rock ; it is cut by the 

 troktolite, and both are cut by an ordinary coarse olivine-gabbro. 

 Now the serpentine and troktolite are so closely united that at the 

 first glance we might suppose the one passed into the other. This, 

 however, is certainly not the case, but the junction is so perfect that 

 they appear, as it were, fused together; while at the junction of the 

 olivine-gabbro and the serpentine there is commonly a line of division 

 or a crack, which is enlarged by weathering. It would be interest- 

 ing to see whether in the Belhelvie ridge there were any indications 

 of a normal gabbro and what was its relation to the other rocks. 

 Professor Judd has also observed the complete sequence of serpentine 

 (or peridotite), troktolite, and olivine-gabbro in the western isles 

 of Scotland. For the last rock to cut serpentine is common. 



One point remains for consideration. What is the relation of the 

 troktolite to the bastite-serpentine ? Do they form part of one 

 and the same eruption, or is the one distinctly subsequent to the 

 other? The difference between the two rocks, notwithstanding the 

 rapid variation which is often exhibited by peridotic rocks, seems 

 too great to render the former hypothesis very pi'obable ; further, by 

 careful search I discovered in one part of the pit a fairly sharp 

 junction between the serpentine with bastite and the troktolite, of 

 which I succeeded in securing specimens. I noticed, however, that 

 in one place the troktolite, probably at a distance of three or four feet 

 from this apparent line of demarcation, changed in the course of 

 about three inches from a variety in which the felspar distinctly 

 prevailed over the olivine to one in which there was more olivine 

 than felspar, perhaps twice as much. In the latter the felspar is 

 almost wholly replaced by secondary products. The greater part 

 of the replacing substance is a minutely granular to slightly fibrous 

 material, which with crossed Nicols changes, as the slide is rotated, 

 from dark to a pale milky or greyish white ; in it are scattered 

 a few granules giving rather bright colours. Much of this might 

 readily be taken for a steatitic mineral, but on testing the spots at 

 the surface of the rock with a knife the hardness appears to be 

 nearly that of felspar. I have examined a slice cut to exhibit the 

 above-mentioned junction. It is rather less than an inch long; about 

 ■| in. being troktolite, the remainder serpentine. The former (except 

 lor the presence of a peculiar structure to be noted presently) is 

 a normal troktolite, with the plagioclase felspar quite distinct, 

 though much cracked ; the latter, consisting mainly of altered olivine, 

 exhibits among this mineral two or three irregular interspaces 

 occupied by a clear substance, which with transmitted light does not 

 differ much from the felspathic portion of the troktolite, but which 

 with crossed Nicols remains wholly, or almost wholly, dark in every 

 position, and is probably steatite. This can be scratched easily with 

 the knife-point, while the colourless mineral of the troktolite is not 

 easily so marked. At the same time the slide does not exhibit the 

 sharp demarcation commonly seen when one igneous rock has 



