Ill 



large shreds of serpentine make their appearance, and the boundaries of these 

 are not sharply separated from the matrix in which they lie. By the increase 

 in the serpentinous portions the amphibolite passes gradually into a rock which 

 may fairly be called a serpentine, (1) though it differs in macroscopic habit 

 from the normal olivine-serpentines. This serpentine is dark in colour and 

 often shows a more or less schistose structure. It is crowded with large 

 irregular silvery scales which are sometimes aggregated into definite bands, 

 and sometimes scattered irregularly through the dark ground mass. Cleavage 

 flakes of the silvery minerals show in convergent polarised light a black 

 cross, the arms of which do not open out into hyperbolae as the stage is 

 rotated. The mineral is therefore uniaxial. With regard to the serpentine 

 the important questions which arise are these. Has it been formed by the 

 alteration of hornblende or did the amphibolite originally pass into an 

 olivine-rock ? Does the transition in space correspond to a transition 

 in time, or have we here another case similar to those described by 



TSCHERMAK ? 



Microscopic examination shows that the rock differs markedly from 

 normal oliviue serpentine. It consists of a transparent green or colourless 

 substance, in which the irregular " mascheiistructur," due to the separation of 

 iron oxides along the original cracks of olivine, is absent. Where the 

 serpentinising process is complete this substance splits up, under crossed 

 nicols, into doubly-refracting bands and isotropic portions. The disposition 

 of the former often gives rise to a structure which reminds one of lattice- 

 work, and strongly suggests cross- sections of a hornblende prism. The 

 central portions of the lattice - meshes are either occupied by isotropic 

 serpentine or by kernels of a vividly-polarising mineral. In the latter case 

 the adjacent grains give simultaneous extinction, thus proving that their 

 present isolation is merely the result of the development of serpentine from 

 the original crystal which they represent. What was this crystal? As we 

 have already seen, the lattice -structure strongly suggests that it was 

 hornblende. This suggestion is reduced to a certainty by the examination 

 of sections in which the two series of serpentine bands are seen to be 

 parallel to each other. In such sections the maximum extinction for the 

 kernels is about 20 ; in other words, the optical characters accord with the 

 view that we are here dealing with, the vertical section of a hornblende 

 crystal which has been largely changed to serpentine. 



The only other conspicious mineral in microscopic sections is the one 

 already referred to as forming such a striking macroscopic feature in the rock. 

 Transverse sections of this mineral are lath -shaped and give straight 

 extinction. 



In order to determine more precisely the nature of this mineral 



(I KA.LKOWSHY describes somewhat similiar relations between ampbibulite uml serpentine 

 in the Eulengebirge (Die Gneiss-formation des Eulengebirges. 1878. p. 43). In this case the 

 serpentine shows maschenstructur " and has been formed therefore by the alteration of an 

 olivine-rock. Olivine occurs as an accessary constituent of the amphibolite ; so that the 

 transition from amphibolite to serpentine may correspond to an original transition from 

 amphibolite to olivine-rock. 



