Mat 11, 1883.] 



♦ KNO^A^LEDGE 



277 



cult to understand ; but there certainly are some which, in 

 spite of being fertilised by insects, do prefer green to any 

 gaudier or brighter hue. They are generally small, but not 

 minute blossoms. Knawel, lady's mantle, and moscatel are 

 but a few which could be named belonging to this small 

 green class. They all seem to be fertilised by little flies, 

 and their honey, when they have any, is secreted quite 

 openly on the face of the flower. There are a good many 

 puzzles of such a sort still remaining even in British 

 botany; and I mention this one only in order to call atten- 

 tion to the kind of question which yet needs an answer at 

 the hands of observing naturalists. 



THE GEOLOGY OF THE ISLE OF MAN. 



Bv W. .Jerome Hakkisox, F.G.S. 



^pHE main fe.iture of the surface of the Isle of Man is a 

 1 ridge running from Maughold Head (S.E. of Eamsey) 

 to the Calf of Man, a distance of thirty miles. The highest 

 point of this rocky backbone just exceeds 2,000 ft. at 

 Snaefell, but Greebah and South Barrule both approach 



Fig. 1. — Geological Sketch-Map of the lale of Man. 



] Skiddaw Slates. 

 • oV Grit in ditto. 

 II Conglomerate. 

 ^B Alluvium. 



EZJD Carboniferous Lime- 



Btone. 

 lZZ3 CJranite. 

 — ' Direction of Dip. 



,'jOO ft From the line of hills so formed, the surface 

 -lopes down on either side to the sea, the width of the Isle 

 at right angles to its mountain axis) being from ten to 

 "«elve miles. A low flat plain, called the Curragh, 

 cupios about fifty square miles north of a line drawn 

 'om Ramsey through Bishop's Court to PeeL 

 In studying the geology of this isolated tract, we have a 



great advantage in the fine sections of the rocks displayed 

 in the clitis all round the coast. Inlan<l, indeed, good ex- 

 posures are not common ; the strata are so overlaid by- 

 beds of sand, clay, and 'peat of recent origin, and the hill- 

 slopes are so well covered by grass, that the liard rocks 

 which lie beneath are only to be examined at wide 

 intervals. 



If a geologist has only two or three days to spend on the 

 island, he should make at once for Castletown, as the 

 neighbourhood of that town atlbrds by far the most interest- 

 ing and varied display of the strata. Within a fortnight 

 or three wciks it is, however, quite possible to examine 

 the entire rocky structure of the island, so far as it is 

 visible. 



The Government Geological Survey has not yet visited 

 Manxland, so that Sheet 100 of the Ordnance Survey Map 

 cannot be purchased "geologically coloured." The late 

 llev. J. G. Gumming did good and careful work among the 

 rocks, and published his results as part of his excellent 

 book on "The Isle of Man" (184S), and also in the 

 "Journal of the Geological Society of London" (vol. II., 

 p. 317). Of recent papers, those by !Mr. Jno. Home in 

 1874 ("Trans. Edinburgh Geol. Soc," Vol. II.), and the 

 late Rev. J. C. Ward in 1880 (Geoloyical ilaga-.iitc, p. 1), 

 are well worth reading. 



SiLiuiAX Formation. — Three-quarters of the surface of 

 Mona is formed of rocks of Lower Silurian age. The fine 

 sections exposed in the cliffs show hard rocks of varying 

 texture, all of which were deposited as mud on a sea-floor, 

 but which have since been so hardened, that the sandy 

 mud is now a rough stone called (jrii ,■ while the fine mud 

 has formed hard shnhs, which are sometimes cleaved, then 

 becoming a true slate. There must be a total thickness of 

 perhaps five or six thousand feet of these Silurian rocks in 

 the Isle of !Man, and yet such appears to have been the 

 paucity of life in the seas in which they were originally 

 deposited, that this enormous thickness of strata has only- 

 yielded a single species of fossil, viz., I'ahi ochorda amjor, 

 once considered to be a fucoid (seaweed), but now known 

 to represent the meandering burrow of a species of marine 

 worm. 



This fossil, and the general similarity of the strata, help 

 us to identify the Lower Silurian rocks of the Isle of Man 

 with the Skiddaw Slates of Cumberland, and again, through 

 them, with the Arenig beds of Wales. A hard band of 

 grit is very noticeable at the base of the Arenig slates in 

 North Wales ; it occurs in the Lake district, and a some- 

 what similar bed, which crops out at Cam Gerjoil (S.E. of 

 Snaefell), and on St. Ann's Head, may represent it in the 

 Isle of Man. The hills forming the central ridge of the 

 island are composed of hard grey slaty shales and 

 flags ; The quarries at South Barrule show true slates 

 of a black hue, upon which, to the west, rest soft black 

 shales. 



Owing to slow and long-continued movements of the 

 earth's crust in bygone ages, these old slaty rocks have 

 been bent into a double curve. A line drawn from south 

 of Ramsey to Crosby (west of Douglas), and thence to 

 Port Erin, will indicate an i-nifir/im'f axis or line, from 

 irhirh the strata dip in opposite directions ; those on the 

 Douglas side inclining at a high angle to the south-east, 

 and those on the Peel side dipping towards the north-west. 

 Beyond (or east of) Douglas, however, from the Dock to 

 the Head, the strata rise up again, and the dip here is 

 westerlj-. At Spanish Head, in the extreme south of the 

 island, bluish flaggy slates lie horizontally, and are 

 traversed by narrow vertical cracks of great depth, called 

 the " Chasms." The strata here probably form the crown 

 of an anticlinal or arch, a position in which they would 



