SCIENCE-GOSSIP. 



259 



THE ROCKS OF THE ISLE OF MAN. 



By Fred. J. Gray. 



(Continued from page 222J 



», . _ inclusions, or micro-dendritic growths, too, are 



Excursion to Foxdale. ,„ . . , , , , 



present, filled with a black opaque mineral, 



\ T Foxdale the Silurian slates are intruded by a 

 granite "boss," called Dun Howe, which has 

 a height of 757 feet above sea level. This lies to the 

 south-east of Foxdale, and it is pierced by the 

 Foxdale lead mines. At this village there are 

 some immense tips of granite of many varieties 

 brought out of the mines, and I was able to obtain 

 a good number of specimens. The principal kind 

 is a light-grey granite composed of white felspar 

 (albite) quartz and mica (muscovite, chiefly). 

 When first brought to the surface, it is exceedingly 

 hard, having to be blasted, but some of it does 

 not weather well, as I saw much which I was able 

 to crumble in my hand. It is, in fact, composed 

 simply of quartz grains and kaolin, the latter being 

 decomposed felspar. The stone is, however, used 

 considerably in the neighbourhood for building 

 purposes, and appears to stand fairly well. The 

 little church at St. John's is built of the white 

 variety, which has weathered to a dull grey. 



This granite varies in many respects, some is 

 red, other pink, green and yellowish green. As I 

 have said, one kind is hard, another soft and 

 crumbly. Some are close-grained, even holo- 

 crystalline rocks, others being coarse-grained, 

 intersected by bands and veins of a greenish 

 mineral. Quartz veins, too, are of common 

 occurrence, one of my hand specimens being a 

 yellowish green granite with a quartz vein about 

 an inch wide running through it. The rocks 

 containing the green mineral soon decompose, 

 this mineral apparently being the cause, turning 

 first brownish green, then red or deep brown. A 

 great amount of it eventually looks like pieces of 

 old dirty brick ; it is as rotten as the white variety 

 previously mentioned. 



The mineral greisen is also found in connection 

 with this '* boss." It is composed of quartz and 

 muscovite. in some parts evenly commingled, and 

 in others the mica is moulded on the quartz, 

 which is the white massive variety. 



A microscopic examination of the white variety 



of the Foxdale granio • to be .1 nolo- 



crystalline ... f quartz, felspar ami mica 



as essential minerals, and gar ;, and 



- as accewori'- 'I be quart 



it the usual s" 



many inclusions, ami 



Js are 

 fairly MUM 



probably pyrites. The quartz, being the last 

 product of consolidation, is never found idiomorphic, 

 but fills up the interstitial space. 



The felspar, as I have already observed, is chiefly 

 "albite," and exhibits examples of twinning on 

 the " polysynthetic " system. Some of the felspar 

 is fairly idiomorphic, but most of it is turbid and 

 cloudy, and appears to be much decomposed, no 

 doubt owing to the large percentage of soda in its 

 chemical composition. It is the decomposition of 

 the albite which produces the kaolin and causes 

 the stone to decay. A great quantity of this 

 kaolin is brought up from the mines, both with 

 the solid matter and pumped up with the water. 

 It can be seen to colour the little stream running 

 from Foxdale to St. John's, a mile or two away 

 from the mines. Zonary banding and the pericline 

 system of twinning may occasionally be observed 

 amongst the felspar. 



The mica is principally "muscovite," but 

 "biotite" is also present. Some comparatively large 

 plates of muscovite are found, generally longi- 

 tudinal sections. It is very easy to observe in 

 some of these sections that mica is an early product 

 of consolidation, several flakes being seen broken 

 across, evidently by pressure from the harder 

 minerals which have later crystallized. The 

 accessories, apatite and zircon, principally occur 

 as inclusions in the quartz, but garnets are very 

 common in the holocrystalline base. One or two 

 fairly large ones of a reddish yellow colour were 

 found, others occurring as small patches of a brown 

 spongy nature. 



In connection with this granitic "boss," there 

 are many veins of aplite or microgranite, of 

 different colours from grey to yellowish brown, 

 of very fine texture, and containing in some cases 

 plates of mica much larger than the other minerals 

 forming the rock. Some of these veins or dykes 

 have undergone, to a small extent, change of 

 structure, doubtless by pressure, having had a 

 somewhat schistose character set up, similar to 

 the l.axey "trap" dyke previously described 

 I' rider tin: mi' rosr ope a thin si in: of the ordinary 

 aplite is seen to be composed of a granular mosaic 

 0I1 leac quart/ and felspai of very small dimena , 



throughout whir h anv.r altered p. n phviilii 1 ry.l.il ■. 



of the same minerals, and large j .1 .ii <-•. or Hakes of 

 Ite, garnots "f a spongy nature and a bright 



red • olour also occurring. I be Rill a again proves 

 itself U) 00 One "I the In .1 product-, ol ronsolida- 



