384 



NATURE 



ySept. 5, 1872 



nelides. For some of the latter he had proposed the genus Con- 

 chicolitcs ; and he restricted Ti'iiliu-iili/es to straight unattached 

 conical tubes. With this restriction the genus may safely be re- 

 garded as Pteropodous, since no Pteropod ha? an irregularly bent 

 or twisted shell, and none can possibly have a shell attached 

 parasitically to foreign bodies. In the present communi'^ation 

 the author founded a new genus for the reception of a fossli 

 which had been kindly sent him by Mr. Edward Orton, of the 

 Geolo"ical Survey of Ohio, after whom he proposed to name the 

 genus OrUniia. This fossil had formerly been referred to Tciita- 

 cuUtcs, and is of common occurrence in the Lower .Silurian 

 (Hudson River group) of south-western Ohio. No doubt can 

 possibly be entertained as the proper reference of this fossil to 

 the tubicolar annclides. Only a single species is known, which 

 the author named 0. conica, and this occurs in the form of coni- 

 cal tubes attached by the whole of one surface to the shells of 

 Brachiopods and other molluscs, Strophonana altcrnafa being 

 the form which is most commonly infested in this way. The 

 sides of the tubes are furnished with strong annular ridges, 

 which die asvay upon the dor al surface, leaving a narrow vacant 

 space or belt of a peculiar cellular character, exhibiting numerous 

 small alveoli, strongly reminding one of the peculiar cellular 

 structure of the tube of Coriuiliti's. From this latter Ortonia is 

 separated by the complete attachment of the tube along one side, 

 and by its much smaller size. From Conc/iicolites it is dis- 

 tinguished by its mode of attachment, and by never growing 

 socially in clustered masses. 



The Rev. Canon Tristram's papsr On the Giology of Moali was 

 then read. After referring to the researches of M. Lartet and 

 others, the author described the general structure of the southern 

 end of the Jordan valley, which, he said, coincided with a great 

 synclinal depression. The lowest rocks exposed are New Red 

 sandstone ; these occur only on the east side of the Jordan, and 

 are there capped by tertiary limestone, resembling that of the 

 " back-bone " of Palestine. Abundance of springs break out at 

 the junction of the limestone and the New Red, rendering the 

 eastern shores of the Dead Sea veiy fertile. On the west side 

 only three springs occur, and, excepting near these spots, the 

 country is barren. Great deposits of marl are heaped against 

 the western banks, but only a little of this occurs on the eastern 

 .side. Many streams of basalt occur on the eastern side of the 

 Dead Sea. These overlie the tertiary limestone, and are, there- 

 fore, of later age than that. The origin of the lava flows is not 

 yet known — no craters were observed in this district. 



To the north-east of the Dead Sea, on the east of the New Red 

 plain, there is a range of hills formed of tertiary Umest(_>ne. 

 Beyond, to the east of this, the Arabs tell of a vast volcanic 

 tract, covered with ruined cities, which is as yet wholly un- 

 explored. 



In the course of the discussion, Prof. Hull remarked that the 

 statements of the author gave a good example of the formation 

 of a valley by disturbance, and he thought that comparatively 

 little was due to denudation. Mr. Topley thought that even 

 if a fault or synclinal ran along the valley, yet the valley itself 

 was still due to denudation. Even if this were not the case, 

 there was the line of hill, or an escarpment of tertiary limestone, 

 on the north-east of the Dead Sea. The westerly continuation 

 of this had been removed by denudation. He saw no reason 

 why the whole of this denudation should not have been subaiirial, 

 the material having been carried southwards down the continua- 

 tion of the Jordan valley before the great depression was proved. 

 All the evidence compels us to believe that the great depression 

 is of extremely recent geological age. 



Canon Tristram, without giving an opinion as to the denuding 

 agents, thought that the valley of the Jordan was marked out, 

 and in great part formed by disturbance. In reply to Mr, 

 Sharp, he said that the Moabite stone was a block of basalt of 

 the country. Many such blocks of basalt are preserved at the 

 houses there. In reply to Mr. Scott, he observed that the great 

 deposits of salt at the southern end of the Dead Sea were of 

 New Red sandstone age. The great saltness of llie Dead Sea is 

 mainly due to this salt being washed down by streams. Salt 

 occurs all along the line wherever the New Red sandstone has 

 been brought up, as in the Sahara and elsewhere. 



On the Trachyte PorpJiyrics of Antrim and Down in Hie North 

 of Ireland, hy Prof. Edw.ard Hull, F.R.S., Director of the 

 Geological Survey of Ireland. 



Trachyte is one of the rarest of the British rocks, and it is 

 as yet uncertain whether it is to be found amongst these islands 



except in the North of Ireland. In this district it was dis- 

 covered and identified by the late Prof. Jukes and Mr. Du 

 Noyer durina; the progress of the Geological survey in the year 

 1867. No description has as yet been published of this remark- 

 able species of volcanic rock, and I propose to give a short 

 account of its characters and relations to the surrounding for- 

 mations as it occurs both in Antrim and Down. 



Trachyte Porphyry of Antrim. — The principal mass forms a 

 group of eminences about four miles to the north of the town 

 of Antrim, called Tardree mountain, Carneaniy Hill {1,043 

 ft.,) Brown Djd Hill and Scolboa Hill. The tops of three 

 of these hills are formed of basalt in beds capping the trachyte 

 rocks, and it is supposed that basaltic sheets enclose the whole 

 of the trachytic district ; though the survey of the district being 

 incomplete the actual limits have not been determined in every 

 direction. 



The mineral constitution of the trachyte is generally uniform, 

 although the relative proportions of the individual minerals 

 occasionally vary. In general, the rock consists of a nearly 

 white or grey felspathic base, with individual crystals of Sani- 

 dine, a triclinic felspar, blebs or grains of smoke quartz, and 

 rarely a little mica. In some places the grains of silica are 

 exceedingly abundant, giving the rock the appearance of 

 Rhyolite or Perlyte as described by Cotta, minute crystal- 

 line grains of magnetite appear in a sliced section under the 

 microscope. It is in this state tliat the iron mentioned in the 

 analysis below probably occurs. 



The rock is quarried as a building stone at Tardree mountain, 

 where it sometimes assumes a columnar structure. A specimen 

 from one of the quarries w.as subjected to an elaborate anilysis 

 by Mr. E. T. Hardmm, of the Geological Survey of Ireland, 

 who gives the following as the constituents ; — * 



Analysis of Trachyte Porphyry, Tardree ijuarry. 



.Silica 76'96o per cent. 



Alumina .... S'loi ,, 



Peroxide of Iron . . 2 '344 ,, 



Lime 7'o64 ,, 



Magnesia .... 0^294 ,, 



Potash 4 '262 ,, 



Soda i-SiS 



Loss by ignition . . 2 •102 „ 



Phosphoric acid . . trace 



99 '943 

 Specific gravity 2 '433 



Relations of the trachytic and basaltic rocks. ■ — During 

 a recent visit I was enabled to ascertain with the greatest 

 certainty the relative position of the trachytic to the 

 basaltic rocks of the district. In the first place, there does not 

 appear to be any passage or graduation of the two classes of 

 volcanic rock into each other, and each having been erupted 

 and spread out in sheets, exhibits a laminated, or bedded struc- 

 ture, which enables the observer to determine their relative 

 positions without much difficulty. Both at Carnearny and 

 Tardree Hills the trachytic porphyry may be observed to dip 

 beneath tlie basaltic rocks of the surrounding country ; and the 

 observations made here and elsewhere tended to show that, of 

 the two kinds of rock, the trachyte is the older. 



On the other hand, both at Carnearny Hill and Scolboa the 

 trachyte seems to have been penetrated by "necks" of later 

 date filled with basalt, from which some portions of the over- 

 lying basaltic sheets may have been erupted. We are not, 

 however, as yet in a position to say whether or not the trachyte 

 is the oldest and lowest of all the Tertiary Volcanic rocks of 

 County Antrim, as its base is nowhere exposed. + 



The events which have taken place in the volcanic history ol 

 this locality appear to have been as follows : — 



At some early stage of the miocene period large masses of 

 trachytic rocks were poured forth from one or more vents, doubt- 

 less accompanied by craters as in Auvergne. After, probably, 

 a long interval of repose new interruptions of basalt and dolerite 

 took place through fissures and small \olcaiiic vents breaking 

 in some places through the trachyte. These later eruptions of 

 basalt may have enveloped the whole of the trachytic masses 

 which have been subsequently laid bare by denudation. The 

 denudation of this region has been very great during postplio- 

 cene and later times ; and to it is due the obliteration of the 



* Jour. Roy. Geological Soc. Ireland. Vol. iii. part i 

 t Messrs. Hull and J. L. Warren. Explanatory 1 

 uf the Geological Survey of Ireland (1S71). 



27. (New Ser.) 

 oir to sheet 36 



