SCIENCE-GOSSIP. 



219 



THE ROCKS OF THE ISLE OF MAN. 



By Fred. J. Gray. 



(Continue! from page 203.; 



Excursion to Castletown. 



"T^HIS town is without doubt the best place on 

 the island to make as a centre for a day's 

 or a few days" excursion. On leaving the town 

 and walking along the shore of Castletown Bay 

 in the direction of Langness Point, the glacial 

 drift is skirted, and in one part in the neighbour- 

 hood of Sandwick a stretch of low sand dunes is 

 found. On passing by these and continuing in the 

 same direction, the first thing of interest seen is a 

 small exposure of diorite of a dark-green colour, 

 and exceedingly decomposed on the surface. This 

 occurs on the shore of the bay, and, as stated, is of 

 quite small extent, looking in fact merely like a 

 very huge boulder. It stands a few feet higher 

 than the ordinary ground surface at that point. 

 On making an examination of a thin slice of this 

 rock under the microscope, the principal minerals 

 contained therein are found to be hornblende and 

 plagioclase. The hornblende is perfectly idio- 

 morphic, exhibits strong polarization colours in 

 dark-red and green, and shows very well the 

 cleavage angle of 124- 30'. About half of a very 

 large crystal comes just on the edge of the section, 

 and being in that position, the other half is 

 unfortunately not there. The plagioclase is very 

 much decomposed, exhibits cleavage well, and also 

 examples of twinning on the albite system. A 

 very plentiful mineral in this rock is epidote, which 

 occurs in comparatively large patches of a light- 

 green colour, seen without the polarizer. This 

 mineral fills a great deal of the interstitial space 

 between the idiomorphic crystals, but does not itself 

 occur with sharply-defined boundaries, and it is 

 often seen lining fissures in the other minerals. 

 • of a light greyish-brown colour also occurs 

 in this rock. Quartz is very sparingly scattered 

 throughout the rock, and what there is of it is in 

 all probability a decomposition product. One or 

 two small cross sections and a few needles of 

 apatite arc seen, and also a few patches and minute 

 cubes of pyrites. 

 Continuing on me thin beds of lime- 



jr arc met with, 



the outskirts of the Carboniferouj series 



isly mentioned. A little further on wc 



see some very interesting beds of conglomerate, 



varying in thick;. 



underlie the iim- . just mention':'!, and 



are in fact the 

 These dip gently of the 



■ 



1 



action of the sea waves. The general appearance 

 of these beds is very striking. They are of a deep 

 red colour and exceedingly rough, the matrix 

 being generally softer than the pebbles contained 

 therein, having been decomposed and worn away, 

 leaving the pebbly matter standing out in all 

 directions, and so giving the beds and low cliffs an 

 exceedingly rugged appearance. The pebbles in 

 this conglomerate are of varying sizes up to six or 

 seven inches long, and are of different colours, 

 most, however, with the exception of the quartz 

 and quartzite, being of a sombre grey hue. The 

 quartz is of the massive white variety, and the 

 quartzite a very fine grained pinkish rock. 



Many pebbles, likewise composed of the fine 

 kind of this conglomerate, are found strewing the 

 beach in this locality, and will themselves in all 

 probability go to form a similar deposit in ages to 

 come. Thus we can see how geology repeats 

 itself, and is always throwing down for the purpose 

 of building something else up. 



On arriving at a disused copper mine, the con- 

 torted clay schist first makes its appearance. This 

 is here of a blue colour, somewhat dull, the strata 

 being inclined very steeply, say sixty degrees, in the 

 opposite direction to the conglomerate, which they 

 underlie. The beds of schist are contorted in many 

 ways, and show well what enormous causes must 

 have been at work to bring about such results. A 

 further walk along this interesting little peninsula 

 and there are found some splendid examples of 

 arches made by the wearing action of the waves. 

 They are fine instances of the destructive agency 

 of the sea, where the cliffs are composed of two 

 different rocks, namely, a thick deposit of con- 

 glomerate on the top and the contorted schist 

 underneath. In this case a tongue of cliff runs out 

 into the sea; the top of this tongue is a deposit of 

 Conglomerate about ten feet thick, the under part 

 to ground level being blue schist inclined at a very 

 high angle At a short distance from the junction 

 of this particular piece of cliff with the remainder, 

 be* " cut right through from the ground 

 surface t" the underside of the conglomerate bed, 

 and forms an opening approximately square, about 

 eight feet high, and ten feel wide This has very 

 beer brought about by the battery of 

 1 ich ilde, bich ba gradually worn the 

 clifl through, the operation being no doubt aided 

 lature I high angle of the schist. 



,\i the extri i n i nd ol the I angness Point the 



assumes a brilliant claret colour, and 



occasionally a reetii Here it i-. Intersected 



