2l8 



HARDWICKE'S SCIENCE-GOSSIP. 



especially the lower, seen on Whiteside and Gras- ' 

 moor, which have been produced by sandy or gritty 

 ■deposits, and the grains of quartz may frequently be 

 distinguished clearly with the naked eye. Again, in 

 a well-marked bed of grit, in the upper part of the 

 Skiddaw series, to be well studied in Great Cockup, 

 north of Skiddaw, in the sides of the southern breast 

 of Skiddaw, and notably at Lank Rigg and Latter- 

 barrow, in the south-west of the Lake District, the 

 grains of quartz frequently approach the size of small 

 pebbles. In all these cases the quartz fragments are 

 more or less rounded, showing that they have been 

 rolled in the water, and are formed of white quartz, 

 such as occurs in veins, or such as may be won from 

 the disintegration of granite. Even in the clay-slate 

 proper, the microscope reveals the presence of small j 

 grains of quartz amongst the aluminous matter. 



In some of the beds of Carboniferous Limestone 

 that wrap round the mountainous tract, quartz occurs 

 both in the form of small pebbles and of curious 

 amorphous masses, very similar to the flint in chalk, 

 but known as chert. In the sandstones, interstratified 

 with the limestone, quartz, in more or less rounded 

 grains, is the chief constituent. In the Penrith sand- 

 stone, of younger age than the Carboniferous, the 

 small grains of quartz are specially interesting, 

 because, in some parts of the sandstone, at any rate, 

 each grain shows the form of a doubly-pyramidal 

 crystal, the crystals being of very uniform size, and 

 their form often not a great deal affected by rolling. 

 Whence these little crystalline particles could have 

 been derived, to form the sandstone, is somewhat of 

 a puzzle. Among the Blue Mountains of New South 

 Wales, Darwin observed a similar case, and remarks : 

 " It is difficult to imagine how these crystals can have 

 been formed ; one can hardly believe that they were 

 separately precipitated in their present crystallized state. 

 Is it possible that rounded grains of quartz may have 

 been acted on by a fluid corroding their surfaces and 

 depositing on them fresh silica ? " The silica in old 

 glass sometimes regains its crystalline structure, as 

 shown by Sir David Brewster in 1840. 



Quartz as a Constituent of Unstratified and 

 Volcanic Rocks. 



( 1 . ) Among Granitic and Granitoid Rocks. — Gene- 

 rally speaking, in true granites, quartz occurs uncrys- 

 tallized, being the last of the constituent minerals to 

 solidify ; it then appears to fill up all the interstices 

 of the other minerals, the felspar and the mica. This 

 is the case with the Skiddaw, Eskdale, and Shap 

 granites in the mass ; though sometimes very locally, 

 or in the form of dykes proceeding from the main 

 mass, the disposition of the quartz is not interstitial, 

 but crystalline. This latter mode of occurrence may 

 be well studied in the quartz felsite of St. John, and 

 notably in the Armboth Dyke. In these cases the 

 quartz and some of the felspar have crystallized out 

 in a felsitic base, and the cross sections of the quartz 



crystals often look nearly square from the unequal 

 development of all the six sides. In the Armboth 

 Quartz Felsite Dyke, the embedded crystals are most 

 clearly seen, and the pyramidal termination of both 

 ends may be well observed. 



(2.) Among intrusive Diorites, Dolerites, cj-y. 

 ( Greenstones). — I know of no cases of quartz occurring 

 in a crystalline condition in the rocks of the Green- 

 stone class. Nor is quartz generally a conspicuous 

 constituent in any form among such rocks. Micro- 

 scopic study of the class, however, reveals the very 

 frequent existence of this mineral in small portions 

 among the other constituent minerals. In some cases 

 its presence may be due to deposition subsequent to 

 the first formation of the rock, and such belong to 

 our next group. 



(3.) Quartz as an Accidental Constituent. — -By an 

 accidental constituent is meant one that forms no 

 essential part of the rock, but has been introduced, 

 perhaps, long after that rock was formed or solidified. 

 Its manner of occurrence in these cases clearly shows 

 that the quartz has been deposited from solution, — 

 water containing silica infiltrating through the rock- 

 mass. It thus fills up cavities, and sometimes replaces 

 other minerals dissolved away. 



Among the volcanic rocks of the district, quartz is 

 very common in this form. In beds of lava, and 

 sometimes in those of volcanic ash, vesicles or long 

 almond-shaped cavities are generally produced by the 

 escape of vapour and gases from the molten or heated 

 matter, on its first eruption from the volcano. Subse- 

 quently, when such lava-beds have been covered up 

 by great thicknesses of overlying strata, the water, 

 which is for ever percolating the crust of our earth, 

 and contains very various mineral substances dissolved 

 in it, deposits these in the cavities and vesicles, and 

 amongst other minerals thus left to fill the vacant 

 spaces quartz is very common, and may often be 

 found filling the same vesicle with the minerals 

 calcite (carbonate of lime) and chlorite. When a 

 large vesicle has been thus filled with quartz, vari- 

 ously coloured and under different physical forms, it 

 sometimes happens that the surrounding rock is 

 subsequently broken up and destroyed, and then the 

 hard kernels of quartz are isolated in the form of 

 agates. Such are the agates of Wallow Crag, 



Keswick. 



{To be continued.) 



BRAMBLES ABOUT LONDON.— II. 



By Dr. E. De Crespigny, Author of " A London 

 Flora." 



iv. V I: 



LLICAULES.— In this section the 

 aciculce and setae disappear : the stems 

 are angular, strong, prickly, and furnished with hairs, 

 which are usually duplicate or fascicled, and spread- 

 ing; orstellately downy and adpressedj a difference 



