G R A 



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G R A 



a provincial term used by miners.) 

 The name given to a group of rocks, 

 being the lowest members of the 

 secondary strata. Sir C. Lyell 

 comprises in this group the Ludlow, 

 Wenlock and Dudley, Horderly and 

 May Hill rocks, the Builth and 

 Landeilo flags, and the Longmynd 

 rocks. The French have changed 

 the name grauwacke for traumate, a 

 word as little euphonic as the one 

 repudiated. Sir R. Murchison 

 eays " It would appear that the 

 Silurian loved to dwell amid the 

 relics of the old greywacke of 

 the Scottish region, as well as 

 along the Welsh border ; and thus 

 I rejoice in having substituted a 

 pleasing name, full of glorious 

 British recollections, for the foreign 

 term, Grauwacke." Thegrauwacke 

 group may be regarded as a mass 

 of sandstones, slates, and conglo- 

 merates, in which limestones are 

 occasionally developed. Sandstones 

 which mineralogically resemble the 

 old red sandstone of the English, 

 not only occupy the upper part, 

 but frequently also other situations 

 in the series. In the lower portions 

 of the grauwacke group, stratified 

 compounds, resembling some of the 

 unstratified rocks, are by no means 

 unfrequent. In speaking of ' ' grau- 

 wacke " Sir R. Murchison says " a 

 name which, until recently, com- 

 prehended every rock from the 

 roofing slates to the beds immedi- 

 ately beneath the old red sand- 

 stone. It may indeed be said that 

 grauwacke was at that time con- 

 sidered the limit, on reaching 

 which all stratigraphical and geo- 

 logical definition ceased. Thia 

 word should cease to be used in 

 geological nomenclature, and it is 

 also mineralogically valueless, be- 

 cause rocks undistinguishable from 

 the so called grauwacke occur both 

 in the old red sandstone and in the 

 coal measures." Sir R. Murchison 

 adds "I might, I believe, assert 



that rock specimens, which many 

 mineralogists would term grey- 

 wacke, may be found in every 

 stage of the geological series, even 

 in the tertiary deposits." Mr. 

 Bake well observes, " Graywacke, 

 in its most common form, may be 

 described as a coarse slate contain- 

 ing particles or fragments of other 

 rocks or minerals, varying in size 

 from two or more inches to the 

 smallest grain." When the im- 

 bedded particles become extremely 

 minute, graywacke passes into 

 common slate. When the particles 

 and fragments are numerous, and 

 the slate in which they are ce- 

 mented can scarcely be perceived, 

 graywacke becomes coarse sand- 

 stone, or gritstone. When the 

 fragments are larger and angular, 

 graywacke 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. The old 

 red sandstone is a graywack, colour- 

 ed red by the accidental admixture 

 of oxide of iron; it possesses all 

 the mineral characters, and occupies 

 the geological position, of gray- 

 wacke. The rock, though composed 

 of substances of various colours, 

 usually exhibits some shade of gray 

 or brown ; it is sometimes of con- 

 siderable hardness, and susceptible 

 of a high polish. Graywacke is 

 often distinctly stratified, but the 

 strata are not usually parallel to 

 those of the subjacent rocks. The 

 common and slaty varieties often 

 alternate with each other, and both 

 are traversed by veins of quartz. 

 This rock is remarkably metalli- 

 ferous ; and its ores occur both in 

 beds and veins. Most of the mines 

 of the Hartz are contained in 

 greywacke. 



Of the fossils of the greywacke^ 

 zoophytes and crinoidea are the 

 most numerous. The trilobite is 

 characteristic of this era, and the 



