2*2 Profs. W. King and T. H. Rowney on the 



Dr. Haughton has appropriately called the rocks we are en- 

 gaged with porphyritic serpentine, on account of the abundance 

 of rude crystalline forms imbedded in its amorphous base. 

 When large and coarsely laminated, these bodies have much 

 the appearance of altered diallage; but in very many cases we 

 have no doubt of their representing another mineral. 



Fig. 1, Plate II. , gives the outline of one of the rude crys- 

 talline forms, which are usually so crushed out of shape as to 

 be rendered undefinable. In this example, however, we have 

 succeeded in determining the usually modified crystal of 

 pyroxene — a short oblique rhombic prism: the observed three 

 faces correspond with those belonging to its front (or back)*. 

 The crystal does not now consist of pyroxene ; on the contrary, 

 its substance has quite the appearance of chlorite, in colour, 

 lustre, and cleavage. Fig. 2 represents a transverse section of 

 another crystal, showing its form and chlorite-like cleavage. 

 In many cases the latter character is rather coarse, which, with 

 a metallic lustre, reminds one of bronzite. At first we ima- 

 gined the crystals to consist of a lamellar allomorph of serpen- 

 tine ; but, whatever their substance may be, there can be no 

 doubt that they are pseudomorphs after pyroxene. 



The Lizard serpentine rock has its counterpart in the north 

 of Italy — the latter being porphyritic, and charged with crushed 

 crystals apparently similar to those just described. In the 

 and south-east of Europe there are igneous rocks strongly 

 suggestive as to the original character of our home rock, viz. 

 the wackite of Schima, Bohemia, which contains both py- 



* The three faces might be mistaken for laterals. What gives rise to 

 this false appearance is, that the two faces forming the bottom;, also the top, 

 (if we may be excused using such non-technical terms, applicable, how- 

 ever, to an oblique solid) of an ordinary crystal of pyroxene are generally 

 unequally truncated, one (say, belonging to the top) being deeper or lower 

 and consequently larger than the other. In rig. 1 the largest and deepest 

 face forms the left slope of the top, and the corresponding one the right 

 slope of the bottom, which peculiarity, it will be observed, causes the solid 

 to appear inclined in this aspect. The front and back of a crystal of pyroxene 

 are often similarly modified (that is, unequally), so that the vertical edges 

 bounding the central face of the front, instead of being opposite to their 

 analogues of the back, are usually opposite to the faces of this part. It is 

 this peculiarity which has produced the vertical ridges and furrows com- 

 monly seen on the crystalline forms in the Lizard serpentinite • for it will 

 be understood that edges when opposite to each other must offer more 

 resistance to pressure than when they are not opposite : hence it is that 

 a face belonging* to the front of these forms is crushed in against an edge 

 belonging to the corresponding or opposite part, the one making a furrow 

 and the other a ridge. These forms cannot have been originally horn- 

 blende, or the observed central face would have been less acutely rhom- 

 boidal, and it would have been lateral : in the usual crystal of hornblende 

 the front and back consist each of two faces; as maybe gathered from 

 the above, each of these parts in pyroxene consists of three faces. 



