330 J. P. Iddings — Columnar structure in the 



Robert Mallet, in a paper " On the Origin and Mechanism of 

 Production of the Prismatic (or Columnar) Structure of Ba- 

 salt," read Jan. 21, 1875, and printed in abstract in the " Pro- 

 ceedings of the Royal Society of London, 1875," also showed 

 that the cracking of a shrinking mass which is cooling from 

 the surface is quite sufficient to account for all the phenomena 

 of columnar structure. He referred the radiation and curving 

 of columns wholly to influences arising from the contour of the 

 surface of the cooling mass, not taking into consideration the 

 effect of different rates of cooling at the surface, which must 

 often vary greatly. He explained the cause of cup-shaped 

 joints in a manner similar to the one here suggested, except 

 that it is not general enough to account for the actual occur- 

 rences, since it requires all the cups to bow in the direction in 

 which the columns advanced during their growth. 



Professor T. Gr. Bonney, in an article "On Columnar, Fissile 

 and Spheroidal Structure," read Feb. 23d, 1876, and printed in 

 the Quart. Journ. of Geol. Soc, vol. xxxii, p. 140, considers the 

 cup-shaped joint a special case of spheroidal parting, and 

 adopts Scrope's and Mallet's views on columnar structure ; he, 

 however, considers fissile or tabular parting as an extreme form 

 of spheroidal, for, as he points out, a plane surface is the same 

 as that of a sphere whose center is at an infinite distance, which 

 leads him to conclude that surface cooling of itself is sufficient 

 to produce tabular parting, in contradiction to the conclusions 

 already adopted that it produces prismatic cracking. This in a 

 measure illustrates the fallacy of explaining certain geological 

 structures and forms by a course of mathematical reasoning, 

 simply because the ultimate forms so derived are similar to the 

 geological forms observed, though the latter were not produced 

 under the conditions imposed by the course of mathematical 

 reasoning, but may have resulted in many cases from wholly 

 different or exactly opposite causes. 



As the rock from the quarries just described presents certain 

 petrographic features which distinguish it from most of the 

 similar igneous rocks in this part of the country, and in a 

 measure accord with its lithological structure, a brief notice of 

 them is here given. Generally the microstructure of these 

 rocks is holocrystalline, formed of lath-shaped, basic feldspar, 

 irregular crystals and grains of augite, grains of iron-oxide and 

 considerable green serpentine or chlorite, which is disseminated 

 through the mass and is evidently the alteration product of a 

 fourth primary constituent. It is probably the drying of this 

 serpentine or chlorite which after a time gives a greenish color 

 to the surface of the rock, which is dark bluish-black when 

 broken. The rock from John O'Rourke's quarry is unusually 

 fine grained and dark colored, and parts of it show little if any 

 change on exposed surfaces. Specimens collected from the 



