86 



GREYWACKE AND GREYWACKE SLATE. 



Slate is regarded as one of the most metalliferous rocks : nearly- 

 all the principal metallic ores have been found in slate, either in veins 

 or beds ; but it is remarkable that flinty slate seldom contains any 

 repositories of metallic matter. Lead and copper are the principal 

 metals found in the slate rocks of England and Wales : they are not 

 so rich in lead as the mountain limestone, but the lead ore in slate 

 rocks contains a larger portion of silver. The killas of Cornw^all, so 

 remarkably metalliferous, is a variety of slate. 



Greywacke and Greywacke-Slate ; German Grauwacke. — This 

 dissonant term, which v^^e have borrowed from the German, the 

 French geologists have exchanged for a name not more harmonious, 

 though more expressive, Traumaie, from the Greek Thrausma, a 

 fragment. 



Greywacke, in its most common form, may be described as a 

 coarse slate containing particles or fragments of other rocks or min- 

 erals, varying in size from two or more inches to the smallest grain 

 that can be perceived by the eye. When the imbedded particles 

 become extremely minute, greywacke passes into common clay-slate. 

 When the particles and fragments are numerous, and the slate in 

 which they are cemented can scarcely be perceived, greywacke be- 

 comes coarse sandstone or gritstone. When the fragments are larger 

 and angular, greywacke might be described as a breccia with a paste 

 of slate. When the fragments are rounded, it might not improperly 

 be called an ancient conglomerate. When rocks of greywacke have 

 a slaty structure, they form greywacke-slate. 



Greywacke has by some of the French geologists been described 

 as a transition sandstone, with a cement either of siliceous earth, or 

 of slate. This definition agrees with the gritstones associated with 

 the upper transition or mountain Hmestone. Where the paste is hard 

 and siliceous, as 1 have observed in the greywacke of Savoy, that 

 separates the primary from the secondary rocks, many of the sili- 

 ceous particles may have been original concretions formed at the 

 same time as the paste ; and where these concretions are all com- 

 posed of quartz, we may infer that such has been their mode of for- 

 mation. In other instances, the fragments are evidently the debris 

 of more ancient rocks, that have been broken down by some great 

 catastrophe, and mixed with more recent beds at the period when 

 they were forming. This mode of formation implies, that a consid- 

 erable period elapsed between the formation of the primary and se- 

 condary rocks. The fragments are always those of lower rocks, 

 and never of the upper strata. In some situations, immense beds 

 of loose conglomerate, composed of large fragments and boulders 

 of the lower rocks, separate the slate rocks from the calcareous for- 

 mations : such conglomerates may be regarded as occupying the geo- 

 logical place of greywacke, and belonging to the greywacke for- 

 mation. 



