74 W. A. E. Ussher — Historical Geology of Cornwall. 



Slate and the Glengariff Grits is an impossibility, on account of the 

 universal pai'allelism between the strike of the beds in the first with 

 those in the underlying Yellow Sandstone, Old Eed Sandstone, and 

 Glengariff Grits. The question of the age of the Glengariff and 

 Dingle Beds, so far as their relative positions are concerned, remains 

 nearly as undecided as in Jukes's time. Jukes found that strati- 

 graphically these rocks were allied to the Silurians, but Salter insisted 

 that the fossils were Carboniferous, and that the rocks should be 

 similarly classed ; at the present time Baily reiterates Salter's 

 opinion. 



Are the Dingle or Glengariff Beds Silurian or Old Eed Sandstone ? 

 — The answer to this question depends on the relative values of, — 

 stratigraphical position and fossil evidence. If the first is most 

 important, then the rocks belong to the Silurian ; but if the second, 

 then they are of Old Eed Sandstone age, a group of rocks that form 

 passage-beds from the Silurian into the Carboniferous. It seems 

 remarkable that Prof. Hull should class the Dingle and Glengariff 

 Beds as Silurians, while he has mapped the rocks of the Curlew 

 Mountains, which are stratigraphically similar, as Old Bed Sandstone. 



It is unnecessary to enter into Devonian geology further than to 

 point out that Jukes' fault has not been disproved, while the Irish 

 geologists who have been in Devon believe in its existence ; also the 

 fossil evidence, so much relied on, seems not to be of much geological 

 value, inasmuch as the species have been collected without that 

 care and precision which can alone render them of use in marking 

 horizons. The localities assigned to the specimens, in the collections 

 chiefly relied upon, are such as Torquay, Chudleigh, etc. ; where two, 

 if not more, distinct groups of rocks are developed. 



T 



IV. — Historical Geology of Cornwall. 



By W. A. E. Ussher, F.G.S. 



(Continued from the January Number, p. 36.) 



(PLATE III.) 



Part 4. — St. Michael's Mount. 



HE best description of St. Michael's Mount, as it now exists, that I 

 JL can find, is by Mr. Wm. Pengelly, F.K.S., 1 as follows: "The Mount 

 is an isolated mass of granite measuring about five furlongs in peri- 

 meter at its base. At high- water it plunges abruptly into the sea, except 

 on the northern or landward side, where the granite comes in contact 

 with the slate, into which it sends veins and dykes, as may be well 

 seen on each side of the harbour. Here there is a small plain 

 occupied by a village, adjacent to which is the harbour, built in 

 1726-7, and, as Mr. Johns, the harbour-master, has been good enough 

 1o write me, capable of receiving ships of 500 tons burthen." Its 

 situation is described as follows : "The distance between the nearest 

 point of Marazion Cliff and spring- tide high- water mark on the 

 Mount is 1680 feet. A tidal isthmus (Hogus) of highly inclined 

 Devonian slate and associated rocks, in most cases covered with a 

 1 Journ. Eoy. Inst. Corn, for 1873, p. 12. 



