364 Geological Society: — 



work, by means of two conical pendulum-seismographs. The ver- 

 tical motion is recorded on the same sheet of paper by means of 

 a compensated-spring seismograph. In details these instruments 

 differ considerably from those described in the Philosophical Maga- 

 zine for September 1881 ; but the principle is the same. 



The time of occurrence of an earthquake is determined by causing 

 the circuit of two electromagnets to be closed by the shaking. One 

 of these magnets relieves a mechanism, forming part of a time- 

 keeper, which causes the dial of the timepiece to come suddenly 

 forward on the hands and then move back to its original position. 

 The hands are provided with ink-pads, which mark their positions 

 on the dial, thus indicating the hour, minute, and second when the 

 circuit was closed. The second electromagnet causes a pointer to 

 make a mark on the paper receiving the record of the motion. 

 This mark indicates the part of the earthquake at which the circuit 

 was closed. 



The duration of the earthquake is estimated from the length of 

 the record on the smoked paper and the rate of motion of the drum. 

 The nature and period of the different movements are obtained 

 from the curves drawn on the paper. 



2. "Notes on some Fossils, chiefly Mollusca, from the Inferior 

 Oolite." By the Rev. G. F. Whidborne, M.A., F.G.S. 



3. " On some Fossil Sponges from the Inferior Oolite." By 

 Prof. W. J. Sollas, M.A., F.G.S. 



4. "On the Dinosaurs from the Maastricht Beds." By Prof. 

 H. G. Seeley, F.E.S., F.G.S. 



March 21.— J. W. Hulke, Esq., F.E.S., President, 

 in the Chair. 



The following communication was read: — 



" On the supposed Pre-Cambrian Eocks of St. David's." By 

 Archibald Geikie, Esq., LL.D., F.E.S. (Part I.) 



The author began by briefly narrating the circumstances under 

 which he had been led to study the geology of St. David's. He had 

 visited the district twice, first in company with Mr. B. N". Peach, 

 with whose cooperation nearly all the field-work was done, and 

 again in conjunction with Mr. W. Topley. The paper was divided 

 into two parts, the first being mainly controversial, and the second 

 descriptive. Only the first part was read. 



Paet I. 

 According to Dr. Hicks there are at St. David's three distinct Pre- 

 Cambrian formations : — the " Dimetian," consisting of crystalline, 

 gneissic, and granitoid rocks; the "Arvonian," formed of felsites, 

 quartz-porphyries, halleflintas, and other highly silicated rocks ; and 

 the " Pebidian," composed of tuffs, volcanic breccias, and basic 

 lavas. He regards the " Arvonian" as later than, and unconform- 

 able to the "Dimetian," and the Pebidian as younger than, and 



