Jan. 30, 1885.] 



♦ KNOWLEDGE * 



83 



late Prof. Nichol, of Glasgow (Eng.), aud some others who 

 seem to have no idea of the real sort of work which is | 

 required in dealing with the architecture of the heavens. 



Struve saw that some impottauoe attaches to the inquiry 

 whether stars of the brighter orders — such for instance as 

 can be seen with a 2inch telescope — are more richly 

 strewn on the Milky Way than elsewhere. Clearly, ac- 

 cording to Sir W. llei-schel's earlier ideas they ought not 

 to be. The i-ange of distance around our sun within which 

 such stars are included, on the assumption of generally 

 equal distribution, falls well within the breadth of 

 Herschel's flat-disc galaxy, and (on that assumption) it is 

 only when we pass far beyond such distances, that we 

 come either on the vacant space bordering the flat sides of 

 our galaxy, or on the mighty vistas of stars along its 

 regions of greatest extension which produce the soft light 

 of the Milky Way. Here then was a general test for the 

 validity of the method which Herschel had found to fail 

 him only in specific instances. 



(To he continued.) 



EAMBLES WITH A HAMMER. 



OTEK CIIAUNWOOD FOREST. 

 By W. Jerome Harrison, F.G.S. 



(Continued from page 23.) 



rilHE great difliculty which besets us as to the true 

 A. geological age of the Charnwood slates is the absence, 

 in their neighbourhood, of any other old rocks whose 

 relations to them we might ascertain. In Wales, for 

 example, we feel positive that certain strata are of 

 Cambrian age, although they contain no fossils, because 

 we see Pre-Cambrian rocks hclow them and Silurian 

 strata aboce them. But the oldest rocks which abut on 

 Charnwood are the Carboniferous series. At Gracedieu, 

 in the extreme north-west of the Forest (about midway 

 between Whitwick and Sheepshed) the Mountain Lime- 

 stone (Lower Carboniferous) rests on the Pie-Cambrian 

 slates. It is here dolomitic, containing carbonate of 

 magnesia in addition to carbonate of lime, but the ordinary 

 fossils of this rock — large shells of the genus I'l-oduclus — 

 are fairly common. There is little, if any, true Millstone 

 Grit above the Limestone, but the superincumbent Coal- 

 measures stretch away to the west and south, forming the 

 Leicestershire coalfield. 



Above the ancient slaty rocks we have described — resting 

 on their upturned edges — and of comparatively recent origin 

 as compared with them, we find thin beds of reddish sand- 

 stone and marl, belonging to the Triassic formation. During 

 the deposit of this red sandstone, Charnwood was evidently 

 a little archipelago, the hill-tops standing as islands in the 

 Triassic sea. In most of the granite and slate quarries these . 

 " red rocks " are seen resting on, and tilling up hollows in 

 the old eroded surface of the slaty and syenitic rocks. 



Lastly (covering every other rock more or less irregu- 

 larly), there is the Drift — the surface deposit of clay and 

 sand full of stones — the result of the action of ice during 

 the last glacial period. Glaciers advancing from the north 

 rubbed over our Charnwood hills, and detached great blocks 

 of rock, which they carried southwards, so that boulders of 

 MountsoiTel granite and of Groby syenite are not un- 

 common round Pvugby, and in Hertfordshire ; while Mr. 

 Lucy has traced similar fragments right across the country, 

 from the Cotswold Hills (where he first found them and 

 determined to track them home) to Charnwood. Dwellers 



in London should search for Charnwood rocks in the 

 boulder-clay in the brick-pits of Fiuchley, on the northern 

 brow of the Tliamcs valley, beyond which the ice did not 

 extend southwards. 



The igneous or crystalline rocks of Charnwood are of 

 considerable commercial importance, the "setts,'' or square 

 blocks, made from them being largely used in the soutli 

 and east of England for road making in towns, while the 

 broken rubble is employed as macadam. 



True granite is only found at Mountsorrel, where the 

 rock is worked on a grand scale, about 700 men being 

 em))loyed. The " face ' of the workings is more than 

 half a mile long, and about 200 ft. in height. It is a con- 

 spicuous object from the Midland main-liue, near Sileby. 

 The other large quarries arc in the xi/' ni'f: at Groby and 

 Markfield, but smaller quarries are being opened at Clifl 

 Hill, Ciarendon, itc. Owing to the hardness of the igneous 

 rocks, they usually stand up as bosses or rounded hills, 

 whose heavy curving outlines contrast strongly with the 

 ridges made of slate, the latter being ever sharp, peaked 

 and shivery. 



On the Survey Map we find Bardon Hill and Birchwood 

 Plantation shown as " greenstone," a term which appears 

 to have been used by the officers of the geological survey for 

 any rock which they believed to be igneous, but, of whose pre- 

 cise nature they were not certain. So, too, all the ridge — 

 three miles long — from Gracedieu to Green Hill is marked 

 as " felspathic porphyry." In all these cases modern 

 research — aided by the microscope — has shown these rocks 

 to be volcanic as-hes, now consolidated and greatly altered, 

 but whose original nature is clearly discernible. Very 

 similar beds occur in that division of the Pre-Cambriau 

 strata of Wales to which Dr. Hicks has given the name of 

 Pebidian, and which in North America are called the 

 Huronian series. To this geological epch, then, all the 

 stratified rocks of Charnwood — the slates, aslies, grits, ic. — 

 may be assigned ; but with respect to the igueous rocks — 

 the granite and syenite — we can only say, with certainity, 

 that they must be nea-er than the slates through which 

 they break. Possibly they, too, may date back to Pre- 

 Cambrian times, but it is not improbable that their out- 

 burst may have taken place in the Lower Silurian age, 

 which, in Wales and elsewhere, we know to have been a 

 period of great plutonic and volcanic activity. 



The granite of Mountsorrel consists of felspar (grey to 

 pink in colour), quartz (glassy), mica (black), and horn- 

 blende (green). It is traversed by narrow dykes of felstone 

 (a compact clear pinkish stone), and by a band of green- 

 stone two yards wide (the "Great Fault" of the workmen), 

 which Prof. Bouncy has shown to bo an altered dolerite, 

 composed of felspar and augite. 



The syenite at Groby and elsewhere is composed of the 

 minerals, felspar, hornblende, and quartz. Just outside 

 Brazil Wood is a knoll of a tough, dark rock (diorite), in 

 which black hornblende is mingled with whitish felspar. 



The syenite of Bawdon (Baldwin's) Castle possibly comes 

 up through the faulted anticlinal. I found its junction 

 there with a coarse grit. As the syenite nears the latter 

 rock its crystals become smaller, and at the actual junction 

 there is a thin band of the igneous rock almost as com[ act 

 as glass. The difference is doubtless due to the more rapid 

 cooling of the syenite where it was in contact with the grit. 

 For the formation of large crystals a slow cooling is, as a 

 rule, necessary. Altogether, Charnwood furnishes many 

 diflicult problems to the geologist, and much work yet 

 remains to be done ere the intricacies of its rocks are 

 finally unravelled. The publication — at no distant date, 

 we hope — of maps of the region on the scale of six inches 

 to a mile, will be a great aid to the student, who vv'ill be 



