416 W. 0. Crosby — Absence of Joint- Structure at Great Depths. 



Carboniferous Limestone thrown inider the Glengariff Grits, then the 

 Coal-measures under the Carboniferous Limestone of the Mendips, 

 and Eastward Dumont's well-proved Belgian inversions, etc., our 

 present subject assumes but a very subordinate place in so grand 

 a system. 



Such sound generalizations as a " contracting crust " with " lateral 

 displacement " through " tangential thrust," now happily rank as part 

 of the common stock of knowledge. No writing has tended to bring 

 this about in a greater degree than Prof. Heim's magnificent work 

 " On the Mechanism of the Formation of Mountains, and Monograph 

 of the Todi-Windgalle group," embod^ang the grand discoveries of 

 Escher von der Linth. 



Fortified by such established facts, we feel unfettered in identi- 

 fying distant out-crops from the sum of their characters, palaeonto- 

 logical, etc., a vera causa being recognized for their appearance in 

 positions the most startling at first sight. 



I cherish the hope of being yet able to publish a geological 

 map of this Calciferous district, including portions of Sheets 22 to 

 25 of the One-inch Survey, having spared no pains to lay down the 

 lines as correctly as the imperfect topography and half- worn-out 

 engraving of the old maps will admit of. 



Note. — This seems a fitting opportunity, since several igneous 

 rocks have been referred to, for saying that Mr. Eutley, of the 

 Geological Survey, has kindly examined and given me a written 

 opinion on many slices of South Devon rocks, which have been cut 

 for me by Mr. Cuttell. They are chiefly from the important lavas 

 of Upper South Devon (Upper Devonian) age that run East and 

 West through Totnes and the adjoining parishes of Harberton, 

 Ashprington, Cornworthy, etc. Whilst hoping to do justice to them 

 in their proper place, I will only now observe (being quite a tyro 

 at the polariscope myself) that there is the strongest concurrence 

 between the verdicts given and the aspect of the rocks in the field. 



V. — On the Absence of Joint-Stb.ucture at Great Depths, and 

 ITS Eelations to the Forms of Coarsely Crystalline Erup- 

 tive Masses. 



By W. 0. CnosBY, of Boston, Mass., U.S.A. 

 ^^ "TT is often said by vein-miners who have worked at great 

 X depths, that the jointed structure of rocks fades away and 

 disappears in the deeper parts of the mines. This, however, is 

 probably a case of the rule ' de non aj)parentibus et non existentibus 

 eadem est ratio.' All the joints near the surface are more or less 

 acted on by the weather. The deeper-seated rocks may be just as 

 much traversed by joints, but they are merely mathematical planes 

 of division, the faces of the blocks adhering as closely as if they did 

 not exist till the weather makes them apparent, or some force tears 

 the blocks asunder." 



This extract from Jukes and Geikie's Manual of Geology ' is 

 1 Thii'd edition, p. 181. 



