278 



o KNOV/LEDGE ♦ 



[May 11, 1883. 



he under great strain, and consequent liability to fracture. 

 (Fig. -'•) 



Fifr. 2. — Strata forming an anticlinal curve or saddle. Tlio strain 

 l-as caused tlie beds to fracture, so forming the fissures C, D, E. 



It is very desirable that these Silurian rocks should be 

 more thoroughly searched for fossils ; grnjitolites ought to 

 occur in them, for forty species are known in the Skiddaw 

 Slates. 



Granite bursts through the Silurians at two points — viz., 

 at Doon, near Laxey, and near Foxdale, about two miles 

 north-oast of South Barrule. In each case the intrusive 

 rock has shattered the Silurian shales of the neighbourhood, 

 and heated water, traversing the fissures so formed, has 

 filled them with various minerals, including much lead-ore 

 {galena), zinc-ore (blende), and a little copper-ore. The 

 lead-ore is richly impregnated with silver. In the year 

 1879, the Laxey mines produced 7,200 tons of zinc-ore, 

 worth £27,000, and 1,200 tons of lead-ore (containing 

 40,000 oz. of silver), value £20,000. In the same year, 

 the Foxdale mines produced 2,750 tons of lead-ore (contain- 

 ing 55,000 oz. of silver), value £37,000. 



Fi:r. 3. — Section at Lanp^ness, showing the conglomerate resting 

 anconformably on the Skiddaw Slates. 



B. Sldddaw Slates. 



A. Conglomerate forming base of Carboniferous Limestone. 

 X Fanlt. 



Lower CAnBONiFEROu.s Formation. — On the west side 

 of the promontory of Langness, east of Castletown, a con- 

 glomerate of varying thickness (5 to 50 ft.), containing 

 many quartzite pebbles, rests very unconformably on the 

 Silurian slates (see Fig. .3). Traced northwards, towards the 

 College, the conglomerate gradually passes upwards into a 

 limestone — the well-known carboniferous or mountain lime- 

 stone. At Peel, beds of red sandstone are seen similarly 

 to rest on the slates, forming a strip about two miles in 

 length, and perliaps 30(t ft. in thickness. These red beds 

 at Peel dip north-west towards the sea, and are overlaid 

 by limestone, which, however, is beneath the water. Both 

 the conglomerate of Langness and the red sandstones of 

 Peel were formerly assigned to th(^ period of the Old Red 

 Sandstone, a formation which Hugh Miller did so much to 

 render famous ; they have yielded no fossils, and it is 



certain that they really form the base of, and belong to, 

 the Carboniferous Limestone. 



The latter rock itself occupies an area of ten or twelve 

 square miles in the extreme south of the island round 

 Castletown. It forms a " happy hunting-ground " for 

 the fossil-seeker, for Gumming oljtained no fewer than 

 222 species of shells, ic, from Poolvash. Tlie strata lie 

 in a trough, or synclinal, dipping towards a common axi 

 (see map). The lowest beds are brownish limestones, with 

 partings of shale, exposed in two quarries at Castletown, 

 and altogether about 200 ft. in thickness. These are over- 

 laid by the light-coloured limestones of Poolvash, while the 

 hard dark-coloured limestone (called black marble), worked 

 in a quarry south of Poolvash, forms the highest known 

 stratum. It is the upper beds which are most fossiliferous, 

 and they contain a remarkaVilc mi.xture of ferns and 

 calamites with marine shells, indicating the proximity of 

 land at the time of their formation. Thin, corrugated 

 shells of the genus Posidonia characterise these upper 

 limestones. 



Although the carboniferous rocks only occur at two 

 points in the Isle of Man (Castletown and Peel), yet, if an 

 elevation of the district were to take ■ place, we should 

 doubtless find the red sandstone and the limestone com- 

 pletely encircling the Silurian strata, but at present the 

 later-formed rocks of Carboniferous age lie under water, 

 except at the two points just mentioned. 



Ljneous Rocks. — The coast-line between Castletown and 

 Poolvash aflbrds most interesting evidence of volcanic 

 action during early Carboniferous times. The Stack of 

 Scarlet is a boss of basalt which tills an ancient pipe or 

 vent ; a dyke of similar rock runs from it in a north-west 

 direction, while beds of volcanic ash are interstratified with 

 the neighbouring limestones. 



Glacial Period. — Beds of clay, sand, and gravel, rest 

 irregularly upon the edges of the Silurian and Carboniferous 

 rocks which form the solid geology of the Isle of Man. On 

 the south of Ramsey, a bluish sandy clay, containing 

 boulders of Scotch granite, is seen resting upon the slates ; 

 it is overlaid by a bed of sand 10 ft. thick, above which 

 is a second bed of boulder clay, 6 ft. thick, containing 

 scratched stones, ifec. A bluish clay full of stones covers 

 most of the limestone in the south of the island ; when it 

 is removed, the surface of the rock beneath is frequently 

 seen to be grooved. All this boulder clay was brought by 

 a large glacier which, during the glacial period, advanced 

 from the south of Scotland, ploughing its way down the 

 Irish Sea, and grinding over the Manx hills — a fact to 

 which their rounded and mammillated outlines still bear 

 witness. 



This period of intense cold was succeeded by a steady, 

 though slow, submergence. Down went the island ! Not 

 a peak remained to tell of its existence, or to aflbrd a refuge 

 for a vestige of that alpine flora which elsewhere contrived 

 to linger on, and which still exists on the Welsh and 

 Cumbrian hills. It is to the lifting action of coast ice, con- 

 tinued during this depression, that we must look for an 

 explanation of the fact that numerous blocks of the Fox- 

 dale granite have been raised more than GOO feet, so that 

 they now lie on the verj' top of South Barrule — a mountam 

 only a mile and a half south of the Foxdale boss. 



]'ost-(!hicial lieds. — The submergence alluded to above 

 was succeeded by an elevation so great, that the entire floor 

 of the Irish Sea appeared above the waters, and the British 

 Isles formed a peninsula jutting out from the western coast 

 of Europe. The flat surface of the Curragh formed a part 

 of this old plain, whose surface was studded with small 

 lakes and peat bogs ; the threat Irish Elk roamed freely 

 over the district, and individuals got bogged and lost their 



