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HARDWICKE'S SCIENCE-GOSSIP. 



the Geographical Distribution of Plants ; the Natural 

 History of Bournemouth ; the Frog, a study on 

 evolution, &c. ; all indicating much mental activity 

 and appreciation of the great thoughts which are 

 moving the scientific world. 



Peridermium Pini. — I regret that so many of 

 those who have written to me for specimens of this 

 fungus should have been disappointed in not receiv- 

 ing a specimen. The demand has this time proved 

 greatly in excess of the supply. Should I be for- 

 tunate enough to procure fresh specimens this season, 

 those to whom I have not sent specimens shall receive 

 some, and those whose specimens were poor shall 

 receive others. — Charles F. IF. T. Williams, Bath. 



The Home of the Potato. — Professor J. G. 

 Lemmon believes he has discovered that the original 

 home of the potato is Arizona. He discovered it in 

 bloom last summer in the Huachuca mountains : by 

 the 1st of September the plants had formed tubers 

 as large as hens' eggs. 



GEOLOGY. 



Fossil Sponges from the Inferior Oolite. — 

 A paper on this subject by Prof. W. J. Sollas, M.A., 

 F.G.S., was recently read before the Geological 

 Society. ' Some fossil Sponges have been described 

 from the Inferior Oolite of the Continent, but hitherto 

 none have appeared in the lists of fossils from this for 

 mation in British localities. The collection of Sponges 

 described by Prof. Sollas was made by the Rev. G. 

 F. Whidborne. The author described n species 

 (6 of which he identified with those already described 

 from continental localities), belonging to 9 genera, 

 and concluded his paper with some general remarks. 

 These Sponges are calcareous, but are considered 

 by the author to have been originally siliceous, 

 replacement of the one mineral by the other having 

 taken place as already noticed by him. The beds 

 in which these Sponges are found bear all the ap- 

 pearance of being comparatively shallow - water 

 deposits. 



The supposed Pre-Cambrian Rocks of St. 

 David's. — The following important paper by Archi- 

 bald Geikie, Esq., LL.D., F.R.S., was lately read 

 before the Geological Society. 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 co-operation 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. 

 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 offelsites, 

 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 unconformable 

 to the " Dimetian," and the Pebidian as younger 

 than and unconformable to both ; and he asserts 

 that the basement conglomerate of the Cambrian 

 system lies quite unconformably on all these rocks, 

 and is in great part 'made up out of their waste. 

 Taking up each of these groups iu the order of 

 sequence assigned to them, the author maintained 

 that the " Dimetian group " is an eruptive granite, 

 which has disrupted and altered the Cambrian strata, 

 even above the horizon of the supposed basal con- 

 glomerate. He described a series of natural sections 

 where this relation is exposed, particularly one on the 

 coast at Ogof-Llesugn, where the conglomerate has 

 been torn off and involved in the granite, and has 

 been intensely indurated, so as to become a kind of 

 pebbly quartzite. No other rock occurs within the 

 granite mass except dykes of diabase, which rise 

 through all the rocks of the district, but are especially 

 abundant in the granite. The veins of finer granite, 

 so general in granite areas, are conspicuous here. In 

 short, whether studied in hand-specimens or on the 

 ground, the rock is so unmistakably an eruptive masf, 

 that the author could not understand how this view, 

 which was that expressed on the Geological Survey 

 maps, should ever have been called in question. The 

 manner in which it has risen across the bedding of 

 successive horizons in the Cambrian series proves 

 that, instead of being a Pre-Cambrian gneiss, it must 

 be much younger than all the Cambrian rocks of the 

 district. The " Arvonian group " consists of quartz- 

 iferous porphyries, or elvans, associated with the 

 granite, and of the metamorphosed strata in their 

 vicinity. Reference was made to natural sections 

 where the actual intrusion of the elvans across the 

 bedding of the rocks could be seen. The " Pebidian 

 group " comprises a series of volcanic tuffs and 

 breccias, with interstratified and intrusive lavas. The 

 author maintained that this group forms an integral 

 part of the Cambrian system as developed at St. 

 David's. It has been broken through by the granite 

 and porphyries, and is therefore of older date. 

 Instead of being covered unconformably by the 

 Cambrian conglomerate, as asserted by Dr. Hicks, 

 the volcanic group is covered quite conformably by 

 that rock ; and seams of tuff are interstratified with 

 the conglomerate and occur on various horizons above 

 it. The conglomerate, instead of being mainly com- 

 posed of fragments of the rocks beneath it, consists 

 almost entirely of quartz and quartzite, only 4 per 

 cent, of fragments having been found to have been 

 derived from some of the projecting lava-islands 

 underneath it. From the evidence now brought for- 



