Prof. T. G. Bonney — On Dana's Classification of Rocks. 199 



have been so energetic, as to have caused such miscellaneous earthy 

 matters as are found to enter into the composition of an impure 

 limestone, like that of Kirkton, to separate into lamina?, and to 

 assume a sort of striped disposition (rubane as it is also named), 

 resembling what I have occasionally noticed in Auvergne, where 

 Tertiary strata have come into contact with volcanic rocks. The 

 strata, for instance, of Kirkton quarry, are composed of distinct and 

 alternating thin laminas, some of them being of remarkable tenuity, 

 variously consisting either of pure calcareous matter, of translucent 

 silex, resembling common flint, or of a mixed argillaceous substance, 

 which ajiproaches to the character of porcellanite, or of ferruginous, 

 or even of bituminous layers, originating probably from vegetable 

 matter. 



"Upon one of these very thin aluminous folia, which I have 

 compared to porcellanite, I observed the impression of a Fern, 

 apparently of a Pecopteris, which was delineated upon it like a 

 painting upon porcelain." 



This fern, which has been named Splienopteris affinis by Lindley, 

 is associated with stems, leaves, and fruits of Lepidodendron, 

 Calamites, and many other plants. Entomostraca are also abundant. 



Our restoration of the appendages made (in 1872) after the 

 American species, E. remipes, figured by Prof. James Hall, may 

 possibly need modification when more materials for their better 

 reconstruction are available. Meantime we venture to draw the 

 attention of Scottish geologists to this very interesting and classical 

 locality in the hope that new light may be afforded thereby as to the 

 nature and affinities of this singular Freshwater Crustacean. 



IV. — On Professor Dana's Classification of Eocks. 

 By Professor T. G. Bonnet, M.A., F.E.S. 



THE two important papers by this accomplished veteran of 

 science, which have appeared in the American Journal of 

 Science (vol. xvi. November and December, 1878) [noticed in this 

 present number of the Geological Magazine pp. 222-225], though, 

 as might be expected, of the highest value, are in one or two 

 respects, as it seems to me, open to question. Professor Dana 

 approaches the subject as a chemical mineralogist : I venture to 

 criticize as a field geologist who checks his conclusions by using the 

 microscope. 



There is, however, one point on which I would first venture to 

 express my hearty concurrence with his remarks — namely, upon the 

 impossibility of drawing hard and fast lines of distinction between 

 rocks belonging to different geological ages. Lapse of time of course 

 will bring about certain mineralogical changes, such as the forma- 

 tion of epidote, viridite, chloritic minerals, and various carbonates, 

 or of hornblende in augitic rocks ; or again, under certain circum- 

 stances, the production of tourmaline, lithia mica, etc. ' In short, the 

 present condition of a rock may be said to be the result of two inde- 

 pendent variables — external agents and time — and (within limits) 

 the same result may come from either of these being large in com- 



