This ſpecies is often called jaſper, but improperly, becauſe it contains no argillaceous earth.
TERRA SILICEA (ſiliceous earth), united to argillaceous and a ſmall quantity of calcareous earth.
Cronstedt Min. § 63. Petroſilex. Chert.
TERRA SILICEA (ſiliceous earth) united to argillaceous earth and a little magneſia.
Cronstedt Min. §66. Feldſpathum. Feld ſpat.
TERRA SILICEA (ſiliceous earth), united to magneſia, mild calcareous earth, fluor ſpar and alſo to the calxes of copper and iron. Chryſopraſius. I have not examined this, but inſert it upon the experiments of Mr. Achard.
To determine accurately the ſpecies of earths is the moſt difficult part of mineralogy, for innumerable analyſes yet remain to be made. But that which now ſeems intricate and obſcure will become plain and eaſy when experiments have been ſufficiently multiplied.