Trof. T. Sterrij Hunt — Elements of Primary Geology. 493 



The structures are of the kind for which Prof. Kjerulf ^ has pro- 

 posed the term cataclastic. 



I doubt whether any true clastic (epiclastic-) rocks occur in the 

 Lizard District south of St. Keverne. 



EXPLANATION OF ILLUSTRATIONS. 



The map is merely intended to enable readers to form some idea of the relations of 

 the rocks in the district especially referred to in the latter part of the paper, and to 

 fix the exact localities from which the illustrations are taken. The actual phenomena 

 are too complicated to be accurately represented even on the 25 inch scale At the 

 point marked A an "eye" of gabbro foliated at the maigin and measuring about 

 five or six feet in length may be seen in the " granulitic " series. Figs. 2 and 3 are 

 reproductions from photographs taken at B. Fig. 5 represents a portion of the clifE 

 face about nine feet in height ; Fig. 6 another portion about four feet. The crumpling 

 of the bands is well seen in the negative, but has not been very successfully reproduced 

 in the figure. Fig. 1 is from a photograph taken at C, where the banded gneissic 

 series and the gabbros are seen in juxtaposition. It represents a portion of the clifE 

 about 8 feet in height. The line representing the boundary between the gneissic 

 series and the gabbro should have been brought to the point C. This figure 

 illustrates very well the manner in which the light-coloured granitic rock veins the 

 dark-coloured diorite. It must be remembered that Figures 1, 2, and 3 represent 

 portions of the same rock mass. If the rocks are igneous at C as seems to be 

 proved by Figure 1, they must also be igneous at A and B. The point C is the 

 point where a granite vein is indicated on the geological map of the district. 



III. — Elements of Primary Geology.^ 



By T. Sterrt Hunt, M.A., D.Sc, LL.D., F.R.S. 



1. TTNDER the general terms of Primary and Original, given by 

 ij Werner and others, to the early crystalline rocks, have been 

 included various mineral aggregates which may be classed under 

 three heads : 1. The great stratiform masses of aqueous origin, 

 essentially granitic, and formed by slow deposition at the earth's 

 surface, which I have called Indigenous rocks. 2. Those lesser 

 crystalline masses which were formed under similar conditions to 

 the last, but within veins or fissures in pre-existing rocks, and may 

 therefore be called Endogenous. 3. Eocks resembling the last in 

 their geognostic relations, and often confounded with them, but 

 distinguished by the fact that they have come into their present 

 positions not by deposition from solution, but by displacement in a 

 more or less fluid or plastic state. These Exotic or erupted masses, 

 whether ancient or modern, are in a sense Primaiy, since they 

 sustain, in one way or another, intimate relations to the more ancient 

 rocks to which this name was first given. 



1 Grundfjeldspsrofilet ved Mj(;)sens sydende, Nyt Magazin, 1885, p. 215. 



2 Some moditicatiou in the use of the term clastic is rendered necessary by recent 

 discoveries. I venture to suggest that it should be applied to all rocks which consist 

 largely of mineral fragments and that we should distinguish between the three types 

 of clastic rocks at present recognized by using the terms epiclastic, cataclastic, and 

 pyroclastic. Epiclastic — Rocks formed of fragments resulting from the breaking up 

 of older rocks occurring upon the earth's surface. Cataclastic — Rocks largely com- 

 posed of fragments produced during the deformation of older rocks by the earth- 

 stresses. Pyroclastic — Fragmental rocks of volcanic origin. The same terms may 

 be applied to the structures which characterize the rocks in question. 



^ Paper read before Geological Section, British Association, Manchester, 1887. 



