118 



JOINTS AND CLEAVAGE. 



Upper 

 Gallert. 



140. 



Mica slate from the immediate neighbourhood of 

 Count, vr.M^ From a foHated rock contain . n / 



Wicklo 



rolite. 

 141, 



Holy wood, County Wicklow 



The 



slate has here been changed into a crystalline mass of 

 felspar, quartz, and mica, and has probably almost undergone 



f 



No 



136 to 141 partly show the gradual 

 alteration or metamorphism of the Lower Silurian rocks of 



"Wicklow as thfiv nnnrnnr>T» +h* »m«U. 



These rocks 



at a distance from the granite are ordinary 

 argillaceous slate. As they approach the granite they become 

 micaceous with highly contorted, foliated, gneissic lamina- 

 tions, and near the junction there become developed in the 

 rock crystals of andalusite, staurolite, and garnet, the effect 

 of metamorphic action on some of the materials which con- 

 stitute the slaty rock in its unaltered state. 



Wall-case 46. 



Arranged and described by A. C. Ramsay. 

 Specimens illustrative op Joints and Cleavage. 



The jointed rocks in this case are not arranged according 



to any special system. There 

 joints, that 



is so little known about 



any systematic arrangement according to 

 definite laws is not yet practicable. Their mode of occur- 

 rence and the geometrical forms they give to the rocks 

 they traverse, are obvious in almost every quarry in 

 rocks of all descriptions, and many of these forms are 

 well represented on a small scale in the specimens. 

 The practical use of joints is well understood by quarry- 

 men, both in quarries where the wedge and lever are only 



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