352 Dr. C. Callaway — Parallel Structure in Rocks — 



Geological Society,^ and in a second communication read before the 

 Society on the 6th of April last. My contention is that in the dis- 

 tricts examined there is no gradation whatever between schist and 

 granite ; but that, on the other hand, the igneous rocks frequently 

 acquire a foliated structure. My chief object in writing, however, 

 is to call attention to a series of articles by Prof. J. D. Dana, which 

 appeared in this Magazine^ in 1881. These papers deserve special 

 notice, not only for the interest of the facts described, but because 

 they present the case for the sedimentary origin of igneous rocks 

 with exceptional force. 



Prof. Dana's theory is expressed in the title of his memoir : — 

 " On a Case in which various massive Crystalline Eocks including 

 Soda-Granite, Quartz-Diorite, Norite, Hornblendite, Pyroxenite, and 

 different Chrysolitic Eocks, were made through Metamorphic 

 Agencies in one Metamorphic Process." The district described is 

 in the township of Cortland, to the north of New York City, and 

 lies on both sides of the Eiver Hudson. Associated with the above- 

 named plutonic rocks are gneiss, mica-schist, and limestone. Minute 

 details of the various rocks are unnecessary for my present purpose. 



The argument of Prof. Dana is arranged under the following 

 heads : — 



" 1. Evidences of more or less Complete Fusion." — The evidence in 

 this section simply goes to prove that the igneous rocks have crystal- 

 lized from a state of fusion. So far from affirming a passage between 

 these and the schists, the author states that at Cruger's Point, at a 

 junction between schist and soda-granite, the former "bears evidence 

 of partial fusion and exhibits other contact-phenomena." If there 

 are " contact-phenomena," there cannot be a gradual transition. 



"2. Evidences as to Condition of Fusion." — Were these rocks fused 

 in situ, or were they erupted from below ? Prof. Dana considers 

 that in the former case the results of fusion " are likely to vary at 

 comparatively short intervals, because sedimentary beds often vary 

 thus." " Sediments . . . are liable to frequent and sudden changes 

 as to material, which igneous outflows cannot imitate." Here I 

 must distinctly join issue with Prof. Dana. Some of the banded 

 gneisses of the Malvern Hills show alternations of different kinds of 

 mineral matter as " frequent and sudden " as of any sedimentary 

 strata in the world. Thin red seams in a black groundmass form 

 a banding as clear and vivid as the stripes in the American flag. 

 But the seams are granite and the groundmass is diorite, and the 

 rock with this parallel banding passes, sometimes abruptly, into 

 ordinary diorite with irregular veins of granite. In illustration of 

 the singular way in which igneous rocks mimic sedimentaries, I 

 may refer to the so-called "conglomerates"^ in the crystalline 

 region west of Galway. Granite veins, intruded into jointed diorites, 

 have lapped round and enclosed joint-blocks in such numbers as to 

 suggest the inclusion of pebbles in sediment. Where the rocks have 



1 May, 1885, p. 221. 



2 Feb. p. 59 ; March, p. 110 ; April, p. 162. 



3 Described in mj^ paper read before the Geological Society on April 6th. 



