154 THE GEOLOGY OF THE LAKE ST. CLAIR DISTRICT. 
clearly to the greenstone being of later date than the sand- 
stone. Supposing the greenstone to be anterior to the sand- 
stone we should have to believe that the greenstone portions 
of Mounts Olympus, Ida, Byron, Eldon Range, etc., are much 
the same to-day as they were in carboniferous times, and we 
should also have to believe that the sandstone which, accord- 
ing to this theory, was deposited round the greenstone peaks, 
has weathered in so remarkable a manner as to form regular 
rings round the greenstone centre. This seems most 
improbable. 
That the greenstone is of later date than the sandstone is 
supported by further considerations. Suppose the green- 
stone were of prior date, and a submergence were taking 
place, on account of the columnar structure of these rocks 
the amount of debris would be very great, and all the shore 
material, shingle, etc., would be almost entirely composed of 
greenstone, and there would be comparatively little sand. 
Now I have not seen a trace of greenstone nor anything like 
it in any of the sandstone or conglomerate examined. One 
would expect that so close to the shore, as, for instance, the 
sandstone of Olympus, must have been deposited, on this 
theory, the material laid down would be almost entirely a 
conglomerate of greenstone. So far as we have seen the 
sandstone is a rather fine or medium-grained silicious one. 
Where conglomerate occurs the included pebbles consist of 
material derived from the older rocks, such as quartz, 
quartzite, jaspar, lydian stone, cornelian, ete. Quartz-por- 
phyry probably also occurs in this conglomerate, as pebbles 
of this rock are not uncommon on the stretches of gravelly 
beach here and there along the lake. Further, against such 
a coast as the greenstone would make, the water would be 
deep, and the angle of deposition of any sedimentary material 
would be considerable, dipping away from the shore in all 
directions. So far as we have seen there is no trace 
of this in the sandstone ; on the contrary, one of 
its most striking features is its horizontality, even close 
up to the greenstone. Again, it seems to me rather 
improbable that a rock material having the structure 
of this greenstone could have survived to such an extent since 
pre-carboniferous times. One of the most noticeable features 
about the mountains in this district is the enormous amount 
of greenstone debris that covers their flanks, showing how 
rapidly the work of denudation is proceeding. And is it not 
likely that if the greenstone were anterior to the sandstone, 
that the lines of junction between the two formations would 
be just where we might expect to get the main valleys, as it 
would be here that erosion and denudation would act most 
rapidly, and it would be rare to find the sandstone abutting 
against the greenstone, as we now find it? 
