GNEISS, MICA SCHIST, AND SLATE SYSTEMS. 661 



marble of Greece and Italy, but lead-grey, pink, green, and yellow varieties occur, and 

 sometimes two or more varieties of these colours are united in stripes, spots, or veins. 

 The texture is variously fine and large-grained, the latter distinguishing the marble of 

 which the celebrated Duomo of Milan is constructed, which was taken from the mica- 

 schist of the lake of Como. The white crystalline limestone used in the arts is found 

 abundantly in the Grecian Archipelago, the isle of Paros and the promontory of Athos 

 being composed of it. Jt occurs also in the Apennines, Alps, and Pyrenees, and of 

 inferior quality in different parts of Scotland, particularly in the isle of Skye. Another 

 subordinate member of the gneiss and mica-schist systems quartz-rock is essentially 

 composed of that mineral. The texture of the rock is either granular, the grains being 

 large and distinct, or arenaceous, the grains ljne and apparently in imperfect contact. 

 The intermixture of other minerals gives rise to varieties, but in all its forms a distinct 

 stratification may be traced, and when micaceous it divides even into thin flags.' Quartz 

 occupies very extensive tracts of country, either alone or in union with other strata, the 

 beds varying in thickness from a few inches to one or two thousand feet. In Sutherland- 

 shire, between Lochs Eribol and Assynt, and in the isles of Islay and Jura, large con- 

 tinuous tracts occur, which have a thickness of 2200 feet in the latter island. These 

 regions have a very peculiar aspect, " the hills being in general conoidal, with a smooth 

 flowing outline and few asperities, though with numerous scattered fragments. The soil 

 that covers them is remarkable for sterility even in this land of barrenness ; whilst their 

 summits and declivities, refusing nourishment to the humblest moss, shine with dazzling 

 whiteness. Of this kind is the conical Stack Balloch-nan-fey, the last remarkable moun- 

 tain on the west coast, whose naked ridge of bright quartz shines in the sun like snow, 

 and which was described by Pennant as marble." 



It is impossible to make an accurate approximation to the thickness of these groups of 

 strata, for the overlying masses prevent their being adequately explored, and cause our sur- 

 veys to fall short of the full amount as to magnitude. But sufficient evidence is attainable 

 to vindicate an estimate of several thousand yards, nay, even several miles, from the 

 charge of exaggeration, when applied to the thickness of these systems. An enormous 

 demand is made upon time for the production of these immense beds, the sedimentary 

 origin of which is proved by the water-worn aspect of their component ingredients, their 

 leaf-like arrangement, and lines of stratification. To disintegrate to any considerable 

 extent a solid rock to transfer the material by a river-current to any oceanic site to 

 deposit it, and consolidate the deposition, are excessively slow operations, requiring the 

 lapse of centuries to accomplish the formation of a thin stratum. We are certain, 

 therefore, that the building up of the gneissic and mica-schist systems, by the abrasion of 

 the granite, and the gradual deposition of the detached matter at the bottom of the 

 ocean, must have required a period, with the vastness of which the mind can hardly 

 grapple, though perfectly insignificant in His view, to whom " one day is as a thousand 

 yeafs, and a thousand years are as one day." Of these two groups, Dr. Macculloch remarks, 

 " The thickness of these strata we know to be enormous, their depths are discovered 

 by geological observations and inferences : that they extend to many miles was also 

 proved. We hate every reason to know, from what is now taking place on our own 

 earth, that the accumulation of materials at the bottom of the ocean, is a work infinitely 

 slow. We are sure that such an accumulation as should produce the primary strata 

 as we flow see them, must have occupied a space, from the contemplation of which the 

 mind shrinks." 



The gneiss and mica-schist systems, with a subsequent formation", belong to Mr. Lyell's 

 class of metamorphic rocks, or strata which have undergone various metamorphoses by 

 proximity to great plutonic masses in a state of fusion. Besides the protrusion of melted 



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