HARDWICKE'S SCIENCE-GOSSIP. 



59 



agglomerates, which had been previously called 

 altered Cambrian beds, and were supposed to have 

 been metamorphosed by the central " Syenitic " 

 ridge. Finally, Hicks established here three systems, 

 to which he gave the names of Dimetian, Arvonian, 

 and Pebidian. 



The Dimetian System consists of a series of truly 

 metamorphosed rocks of a curious character. True 

 foliated gneisses are rare, but the rocks are massive 

 in character ; they consist of compact granitoid 

 rocks (granitoidite), with much quartz, and some 

 felspar, little mica and some chlorite. A micro- 

 scopic examination of these has proved them to be 

 elastic or fragmentary in their origin. Besides these, 

 there are quartziferous breccias, quartz schists, quart- 

 zite, one band of crystalline limestone, and some 

 green rocks which appear to have been inter-bedded 

 lavas of a basaltic character. 



The Arvonian System of Dr. Hicks appears to be 

 everywhere separated by faults from the Dimetian, 

 and presents some curious varieties of rocks. Among 



Fig. 46.— Quarry south of Rag^edstone Hill (Holl). a, Holly-bush sandstone ; 



B, gneissose rock. 



these what the Swedish geologists call HliUeflintas 

 are conspicuous. These are altered silicious rocks, 

 which appear to have been formed from the denuda- 

 tion of highly silicious lavas and ashes. In addition 

 to these, however, quartz-porphyries or felsites, true 

 volcanic rocks or acidic lavas occur in some quan- 

 tity, often showing spherulitic structure. 



The Pebidian System is found in unconformable 

 contact with the Dimetian, and faulted against the 

 Arvonian rocks. It consists chiefly of volcanic rocks, 

 agglomerates, conglomerates (by stratification and 

 rounding of fragments), ashy beds, hornstones, and 

 what, for want of a better name, are called imperfect 

 ashy schists of green, grey, and purple colours. One 

 contemporaneous felstone lava appears on the coast 

 west of Porth Liskey, where the beds are well ex- 

 posed. The basement conglomerate of the Cambrian 

 System rests unconformably on these rocks, and 

 contains pebbles of Pebidian rock, thus most indis- 

 putably proving the order of succession and age of 

 these rocks. 



(To be continued.) 



RECREATIONS IN FOSSIL BOTANY. 



(L YGINODENDRON OLDHAMIUM.) 



By James Spencer. 

 No. VIII. 



THIS very singular but most beautiful fossil plant 

 was originally described by the late Mr. 

 Binney, F.R.S., from specimens obtained from the 

 neighbourhood of Oldham, under the name of 

 Dadoxylon OldJiamium. 



At that time this plant was generally thought to 

 have belonged to the conifers;, and to have been the 

 representative of that family of plants in the forests 

 of the coal-measures. 



Some time after Mr. Binney's paper appeared, 

 another paper on this same plant by Professor 

 W. C. Williamson, F.R.S., was published in the 

 Transactions of the Royal Society. In that memoir 

 the author clearly demonstrated, that Mr. Binney's 

 plant was not a dadoxylon, according to Brongniart's 

 description of that genus. He also 

 showed that its structure was so very 

 peculiar and distinctive as to warrant 

 him in placing it in a new genus; 

 and as one of its distinguishing 

 features consisted in the reticulated 

 character of the vessels of the woody 

 cylinder, he gave to the new genus 

 the name of Dictyoxylon, still how- 

 ever retaining Mr. Binney's specific 

 name ; hence, throughout the greater 

 portion of the memoir, our plant re- 

 joiced under the new name of Dicty- 

 oxylon Oldhamium. 



Towards the end of the memoir, 

 the author again changed the name 

 of the plant for the following reason : upon ex- 

 amining a fine series of sandstone fossils in the 

 Liverpool Museum, Professor Williamson thought 

 that some of them were the casts of similar plants 

 to his new genus Dictyoxylon. He subsequently 

 learnt that similar sandstone fossils had been de- 

 scribed by a Scotch gentleman of the name of 

 Gourlie, under the name of Lyginodendron Lands- 

 burghii. Upon reading Mr. Gourlie's description, 

 he became confirmed in his conjecture, viz. that 

 Lyginodendron and Dictyoxylon were only different 

 forms of the same genus. Therefore following the 

 rule laid down as one of the canons of science, 

 namely, that the oldest name should claim precedence, 

 he generously gave up his own name of Dictyoxylon 

 in favour of the older name of Lyginodendron. But, 

 as there was no proof that the Scotch plant was 

 specifically identical with the English one, he still 

 retained Mr. Binney's specific name for the Oldham 

 plant, so that the correct name for this plant is now 

 Lyginodendron Oldhamium. 



The Oldham district has long been noted for its 



