l66 MINERALOGICAL ACCOUNT 



appearance and extent of one of these slips is very- 

 distinct, in a small creek called Tury-gio. 



SLATE-CLAY. 



This rock occurs very sparingly in this island. 

 It sometimes makes its appearance between beds 

 of sandstone, and at Forewick it exists in thick 

 beds. It is often mixed with mica, and then passes 

 into slaty-sandstone. 



AMYGDALOID. 



The beds of this rock are of considerable thick- 

 ness. Its basis seems to be a claystone, of a grey- 

 ish-black colour. It contains numerous cavities, 

 filled with different crystalline minerals, as calc- 

 spar, heavy-spar, fluor-spar, and quartz. As these 

 substances are liable to fall out by being decom- 

 posed, the empty vesicles then give the stone the ap- 

 pearance of vesicular lava. These vesicles are some- 

 times filled with calcedony, or with calc-spar, and 

 in company with these, crystals of quartz, heavy- 

 spar, and fluor-spar are likewise found. When 

 all these substances occurred in the same cavity, 

 they were arranged in the following order. The 

 calcedony covered the cavity nearest the rock, the 

 quartz was imposed upon the calcedony, and co- 

 vered with the calc-spar : The heavy-spar rested 

 on the calc-spar, and the remaining cavities of the 

 vesicle were filled with fluor-spar. The crystals of 

 the calc-spar were often rhomboidal and some- 



