284 Profs. W. King and T. H. Rowney on the 



condition. Very frequently green serpentine occurs in iso- 

 lated pieces of various forms (often angular), and of all sizes 

 down to that of a pin's head, separated by a paler-coloured 

 variety : the latter has the appearance in many cases as if it 

 had run into the separations. 



In most cases the pale-coloured mineral is saponite, which 

 differs from serpentine simply and essentially in containing a 

 multiple more of water; nevertheless saponite is generally 

 taken by mineralogists to be a chemically changed product. 

 We may therefore safely look upon the Cornish examples 

 under notice, considering their associations and mode of occur- 

 rence, as having originated from serpentine through the latter 

 becoming charged with additional water *. 



Assuming the conversion of serpentine into saponite, are we 

 to consider that the change is never carried further than the 

 production of a mineral silicate ? that this latter cannot be re- 

 placed by some other mineral with a different acid ? This would 

 be in opposition to the teaching of the numerous instances 

 known as pseudomorphs, and contrary to the opinion we have 

 expressed on former occasions. The evidences hitherto adduced 

 by ourselves are with difficulty appreciated, on account of their 

 requiring the most careful microscopic observations. We are 

 now, however, enabled to bring forward evidences which 

 require no more than a hand magnifier to be observed with 

 their full meaning. 



One most distinctive peculiarity which separates the Lizard 

 serpentinite from a number of other rocks of the kind, occur- 

 ring in other places, is the absence in it of the variety called 

 ophite : consequently the intimate association of calcite and 

 dolomite with serpentine, as in the latter rock, is exceedingly 

 rare in the Lizard. The few exceptions we are acquainted with 

 were only obtained after considerable delay and difficulty. 



In a few of our specimens showing the serpentine passing 

 into saponite, the latter mineral is occasionally affected with a 

 double divisional structure — an imperfectly developed form of 

 cleavage, its two directions intersecting each other obliquely 

 and at an acute angle f- In certain places a mass of saponite 

 is defined on one side by a zigzag edge ; next to which there 

 is a mass of calcite containing a number of ridges, in the shape 

 of the figure 7, formed of saponite, separated from the zigzag 

 edge, and having their two limbs strictly parallel to the two 

 directions of the latter (see fig. 5). Besides, the two direc- 



* Serpentine consists of Silica 43, Magnesia 44, Water 13. According 

 to Haughton, Cornish saponite contains Silica 42-28, Magnesia 2970, Alu- 

 mina 7-21, Water 18-92. Serpentine, it must be noted, often contains 

 Alumina. 



t Serpentine is occasionally affected with a similar cleavage. See 

 Proc. Roy. Irish Acad. vol. x. pi. 43. fig. 8. 



