REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. 101 



although no uralitisation can be strictly said to be observed, there exists, none the less, 

 an orientation of the hornblende upon the augite nucleus. In fact, it is noticeable that 

 a section of hornblende enclosing several nuclei of augite differently oriented (which 

 could not therefore be parts of one individual) is a unique crystalloid. The cleavages 

 are common, and so are the optical properties for each point of the section. It is thus 

 possible to follow one of the crystalloids of hornblende a long distance from the augite 

 nuclei which gave rise to it. The pleochroism of this hornblende is — 



y > P = a 



yellowish brown. yellowish. 



Biotite must also be mentioned as a constituent mineral of the rock. It is often 

 enclosed in hornblende, and may be considered as a secondary product. Rather large 

 sections of magnetite also occur. The grains of magnetite are also surrounded by a 

 very narrow greenish zone of hornblende, as if the matter which gave origin to the 

 latter had permeated the entire rock. 



From the foregoing description it appears that some of the rocks from the " stone 

 rivers " of the Falkland Islands are amphibolised diabases, of which they present a very 

 remarkable type. 



B. — Notes on some other Rocks from the Falkland Islands. 



The following description relates to other crystalline or clastic rocks collected at the 

 Falkland Islands. One of the most remarkable displays large scales of hornblende, 

 which may measure as much as a centimetre, and between them grains of felspar and 

 quartz occur. In structure and mineralogical composition it is a diorite. It contains 

 large patches of felspar, which appear under the microscope as sections of irregular 

 outline. In some cases no trace of twinning is perceptible, and then the felspar 

 resembles orthoclase ; but other examples, where decomposition has also reached a more 

 or less advanced stage, show polysynthetic lamellae, although usually not many. This 

 characteristic would serve to class the felspar with albite ; it is always difficult to 

 determine the magnitude of the angle of extinction, on account of the small number of 

 sections presenting hemitropic lamellse, still, by measuring the double angle, values of 

 about 6° to 10° were found. These large felspar patches are altered into kaolin, and 

 penetrated by rows of epidote grains along the lines of cleavage. The hornblende, the 

 large crystals of which are irregular in outline, shows the characteristic extinctions of 

 this species. The pleochroism is — 



y > /? > a 



yellowish brown dirty green yellowish 



Black mica occurs as inclusions in the hornblende, and grains of epidote also appear 



