Julien.] 146 [December 6, 



1. Chalcedonic. In this process all the constituent silicates 

 of the rock are decomposed, the bases sometimes remaining as 

 reddish brown, soft ochreous grains, and sometimes completely 

 removed. The silica entirely remains, generally as a white or 

 yellowish chalcedony, passing into white, yellowish or reddish 

 chert. When all the bases have disappeared, and the chalcedony 

 remains as an exceedingly cellular mass of thin scales and plates, 

 parallel or anastomosing with the greatest irregularity, a chalce- 

 donic schist, or siliceous sinter is the result, often bearing some 

 resemblance to a buhrstone. 



2. Hornblendic. In this process, microscopic spicules of 

 greenish actinolite first become more or less abundantly inter- 

 spersed among the olivine-grains. Other varieties present actin- 

 olite-grains visible to the eye ; and these may predominate until 

 the alteration becomes complete. The final result is a green 

 actinolyte-rock or schist, or grayish-white amphibolyte or tremo- 

 lyte-schist, which may be fine grained or very coarse, consisting 

 of huge fibrous masses of grayish-white amphibole, 2 to 3 deci- 

 meters in length, crossing each other in the greatest confusion. 

 Even among these coarse masses, where the conversion and disap- 

 pearance of the dunyte seems complete, a few grains or small 

 bunches of unaltered olivine may be sometimes found in the 

 interstices, on a fresh fracture of the rock. 



3. Talcose. The development of the talc-scales throughout a 

 dunyte is brought about in two ways : by the conversion of the 

 oli vine-grains, partially or completely, into talc, which either 

 envelopes them as a microscopic crust, with an ochreous core 

 (i, e. the separated iron-oxide), or has crystallized out in scales 

 among the interstices of the olivine-granules ; or again, by the 

 alteration of the actinolite-fibres and grains into talc, which is 

 then seen as fibrous pseudomorphous films or scales within the 

 cleavage-planes of the actinolite, or entirely replacing it. The pro- 

 cess has often attacked the rock in both ways in the same mass, and 

 has resulted in the production of talcose dunyte, talcose actino- 

 litic dunyte, talcose amphibolytes and actinolitic steatyte and 

 talc-schists. One novel variety is a white granular steatyte, 

 in which the granules are pseudomorphous after those of the orig- 

 inal olivine, a rock often denominated a " white serpentine." 



4. Ophiolitic. In this common process, well shown near 

 Bakersville, Webster, etc., the olivine has suffered alteration in 



