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SCIEXCE-GOSSIP. 





GEOLOGY 



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EDWARD A. MARTIN. F. 



A Lower Carboniferous Island. — There are 

 still some geologists who firmly believe in the 

 permanence of onr ocean basins and continental 

 areas, and who refer ns for the date of their origin 

 to the earliest years of the separate existence of 

 our globe as. a solid body. They contend that a 

 period will only be placed upon them when the 

 globe itself comes to an end. but even they recog- 

 nise that the edges, at least, of our continents play 

 "see-saw," so to speak; the countries bordering 

 the ocean at one ti me being submerged beneath 

 the waters themselves, and at others raised far out 

 of reach of the waves. The ocean bed which now 

 closely surrounds the land has in times long past 

 also partaken of this see-saw movement. Once it 

 formed dry land: ar other times it approached 

 submergence, forming those raised beaches which 

 are so frequent around our coasts : finally it sank 

 into the trough of the sea, allowing the waters of 

 the ocean to roll where formerly was dry land. 

 The Atlantic Ocean is considered to have at least 

 had its origin prior to the deposition of the Lower 

 Carboniferous, and this view is supported by Prof. 

 Hull and Mr. A. J. Jukes-Browne. All agree that 

 the Atlantic must by that time have been in 

 existence, in order to supply the necessary sedi- 

 ment and other material for the building-up of 

 that formation. According to Jukes-Browne, the 

 sea covered in Lower Carboniferous times the 

 larger part of Great Britain, excepting the rezions 

 beyond the Highlands of Scotland. By deduction 

 this may have been an easterly extension of the great 

 Atlantic, and our Country was then in one of the 

 downward throes in the great game of continental 

 see-saw. In the midst of the sea which then 

 covered England Mr. Jukes-Browne has ingeniously 

 inferred the existence of a large island of "irregular 

 shape, which had its centre in St. Georse's 

 Channel, touching Ireland. Scotland, and Wales, 

 and pushing an arm out to the east through Wales 

 inland into Shropshire: this land approximately 

 occupying the position of the water area of the 

 present day. The existence of this island is mani- 

 fested by the series of shore-deposits and con- 

 glomerates which are found around its ethre. 

 The thinning northward of the Carboniferous 

 Limestone in Monmouthshire and Gloucester is 

 evidence of the shallowing of the sea as approach 

 was made to the island: and a similar thinning 

 occurs throughout the South Wales coalfield. 

 Passing across the water to Ireland, we find 

 Carboniferous rocks in Wexford, which have 

 hsen judged to have been accumulated in a 

 narrow bay. Northward these rocks occur 

 throogh Eildare: whilst at Howth, Swords, 

 and Rush there is ample evidence of the close 

 proximity of land. Thus have been traced a 

 southern and a western side to our island. Pro- 

 ceeding, we are reminded of some thick beds of 

 conglomerate with Ordovician pebbles between 



Bush and Skerries, in one place there being indeed 

 boulder beds, formed at the base of the early cliffs 

 which bounded the island, and where the fall 

 blocks had been enveloped in the grey Carbonifer- 

 ous Limestone which was then in the mak: _ 

 Still farther northward Mr. Jukes-Browne finds 

 that around Drogheda the Lower Carboniferous 

 strata appear to have been laid down along a 

 gradually shelving shore, whilst in the north- - 

 of County Down there are limestones associated with 

 red shales and sandstones which are probably si 

 beds of Upper Limestone age. Passing across into 

 Scotland, we meet the basal conglomerates of 

 Lanarkshire, the so-caUed Calciferous Sandstone 

 being also contemporary with the lower portion of 

 the Carboniferous Limestone of England. If we 

 now draw our island boundary southward towards 

 Wales again, we find in North Wales a consider- 

 able thinning-out of the Carboniferous Limestone 

 towards the south-east, whilst it is entirely absent 

 from the Shrewsbury district. There we are ap- 

 parently at a point which the Lower Carboniferous 

 sea never covered : and where, so far as actual 

 evidence goes, we have reached the eastern limit of 

 the island. Just within the limits of the sea we 

 find the basement bed of lava and volcanic ash. on 

 the east side of the Wrekin, and this may have 

 been thrown up under water by volcanic agencies. 

 In the land area of South Staffordshire we find the 

 edges of the ancient Silurian rocks, worn, denuded, 

 and lying bare, until the Coal Measures came to be . 

 borne in upon them ; whilst in Warwickshire the 

 Cambrian are similarly exposed ere the Coal 

 Measures were thrown down upon them, both areas 

 being free from the marine agencies at w 

 during Lower Carboniferous times. Again, the 

 Coal Measures and Millstone Grit rest, in Shrop- 

 shire, unconformably on Old Bed Sandstone and 

 Silurian rocks, these not being submerged till late 

 in the Carboniferous period. The deduction of the 

 former existence of this large island in Lower 

 Carboniferous times, from the nature of certain 

 local deposits and the directions in which they 

 thin out. is interesting as showing what geology 

 can do in the way of building up pictures of ancient 

 land and sea areas by a study of the physical 

 relations of the rocks. — E. A. Martin. 



The Salt Lake of Larxaca. — In a recent 

 paper read before the Geological Society Mr. C. V. 

 Bellamy described the Salt Lake of Larnaca, 

 Cyprus. The lake occurs in a basin shut off from 

 the sea, its deepest part being about 10 feet below 

 sea-level. The barrier between the salt lake and 

 the sea is made of stiff calcareous clay, associated 

 with masses of conglomerates resting on plasti 

 clay, that on watery mud, and that again on stiff 

 calcareous clay. The sea-water appears to per- 

 colate through the highest deposits, meeting with 

 checks in the conglomerates, and thus reaches the 

 basin somewhat slowly, where it is evaporated by 

 the summer heat until it deposits its salt. Artificial 

 channels have been made to carry the flood-water 

 from the land direct to the sea, so that it does not 

 dilute the brine of the lake. The rainfall in the 

 catchment area round the lake is at the most only 

 enough to supply 223,000,000 gaUons ; and as the 

 lake contains 480,000,000 gallons when full, the 

 balance of 257.000,000 gallons must be derived 

 from the sea. The salt harvest begins in August, 

 at the zenith of summer heat, and it,is reported that 

 a single heavy shower at that time of year suffices 

 to cause its ruin. — E. A. Martin. 



