466 Notices of Memoirs — Dr. T. Sterry Sunt — 



Notwithstanding that the contrary had been expressed, he could not 

 but strongly suspect that they were actually the remains of large 

 corals. He had, however, succeeded in obtaining some true fossils 

 from portions of the Innis Lower Limestone, that had scarcely 

 xmdergone any change. He had not had time to examine them as 

 closely as he would have wished, but they appeared to be Caradoc 

 Bryozoa from the schists of Donegal. This was the first example, 

 as far as he could ascertain, of an undoubted fossil having been 

 detected in these limestones. The fact may be taken as evidence 

 that these deposits and their argillaceous and siliceous masses are 

 of Lower Silurian age, and it seems highly probable that the more 

 intensely metamorphosed rocks in the north-west division of 

 Donegal belong to the same geological period. 



X. — The Origin and Succession of the Ckystalline Eocks.'^ By 

 Prof. T. Sterky Hunt, LL.D., F.R.S. 



AS a preliminary to a statement of the results of many years of 

 study of the crystalline rocks in North America, the author 

 proceeded to consider the question of their origin, which is still a 

 subject of debate between Plutonists and Neptunists. The crys- 

 talline silicate rocks naturally divide themselves into three groups — 

 namely, those indigenous stratified formations which have been 

 called primitive, or primary ; those masses to which, from their 

 relations to contiguous rocks, geologists assign an exotic origin, and, 

 in accordance with a generally accepted theory, have agreed to call 

 igneous or plutonic ; and a third and distinct group of rock-masses 

 which, though like the last, clearly posterior to those encasing them, 

 are now, by most geologists, admitted to be of aqueous origin. This 

 third group includes metalliferous lodes, and various other crystalline 

 veinstones, and is conveniently designated endogenous. It is not 

 always easy to distinguish between the rocks of these three groups ; 

 there are not wanting those who have assigned an igneous origin to 

 metalliferous lodes, and many still confound endogenous granitic 

 veins with the mineralogically similar plutonic granites. In like 

 manner, the distinction between the latter and the stratified granitoid 

 gneisses is frequently not very apparent. That the movement of 

 flow in extravasated plutonic rocks may give to the constituent 

 minerals a stratiform arrangement, is a fact of which both exotic 

 granites and doleritic dykes and masses afford illustrations. More- 

 over, the arrangement due to successive depositions upon the walls 

 of a fissure may give to an endogenous mass a structure which 

 simulates that of a sedimentary rock, and imparting to granitic 

 veinstones a resemblance to gneiss ; while a laminated structure 

 sometimes results from the arrangement of the crystals developed in 

 a cooling mass. Hence there are not wanting those who include 

 under the head of plutonic rocks not only the clearly marked exotic 

 granites, dolerites and diorites, but the granitoid gneisses, the massive 

 bedded greenstones, and likewise the more schistose rocks with which 

 these gneisses and greenstones are often so intimately associated that 

 1 Eead before the British Association, Section C, Dublin, August 19, 1878. 



