THE TRAP DYKES OF THE ORKNEYS. 875 



Prismatic jointing i& well developed, and the flags for a few inches from the margin are 

 slightly baked and hardened. 



Sections from the centre rarely contain the large phenocrysts of hornblende so 

 striking in the hand specimen, as their perfect cleavage renders them exceedingly 

 apt to be shattered in grinding. They do not appear to be idiornorphic, their outlines 

 being rounded and irregular. They are not markedly zonal, but have sometimes a 

 slightly darker border. Twinning is common, and the usual hornblende cleavage very 

 perfect. The only enclosures observed were calcite, apatite, and magnetite. Extinction 

 and pleochroism are the same as in the smaller crystals. That olivine was an original 

 ingredient is proved by the presence of pseudomorphs with the outlines of olivine in 

 specimens the augite of which is quite undecomposed. The serpentine is often covered 

 with a meshwork of magnetite, and mixed with more or less calcite. The augite pheno- 

 crysts are the most numerous (PL I., fig. 3). They axe of rather a pale greenish variety, 

 and perfectly idiornorphic, showing in the prism zone the faces 110, 100, 010, and 

 terminated apparently by 111 and 10T. The prismatic cleavage is highly perfect, and 

 there are in addition numerous irregular cracks. The enclosures are apatite, magnetite, 

 olivine (altered into serpentine), calcite, and occasionally a minute patch of hornblende. 

 The crystals are sometimes twinned on 100. An extinction angle up to 43° was 

 measured. Between crossed nicols there is neither zonal nor hour-glass structure. 

 Decomposition begins at the edges and in the strongly marked cracks, and proceeds 

 gradually over the mineral, so that in early stages there is a perfectly fresh centre, 

 while an outer zone, "3 mm. or more in depth, is passing into deep green chlorite. The 

 increase of volume due to hydration produces a series of fissures perpendicular to the 

 surface of the crystal which make this border very evident. Its inner limits are usually 

 so sharp as to suggest that it is due to an outer layer of slightly different composition, 

 and more liable to alteration than the central mass ; and, indeed, where least altered, 

 this border appears to have a more brownish colour. In other sections the mineral is 

 entirely altered into a pale chlorite mixed with finely divided calcite, and covered with 

 a fine dust of magnetite, with anatase or rutile. Even in these pseudomorphs an outer 

 zone is clearly distinguishable from the central part. Magnetite is abundant in large 

 rounded masses, and in small sharply angular crystals disseminated through the other 

 minerals. Around the larger crystals there is usually a number of smaller grains, 

 radiately disposed, and mixed with biotite. Pyrite is rather common, and the presence 

 of ilmenite is indicated by occasional traces of alteration into leucoxene. Biotite in 

 small scales of deep brown colour and very intense pleochroism is frequently adherent 

 to the iron oxides, and is found in small quantity scattered through the other minerals. 

 There are numerous long delicate pointed crystals of apatite. 



The groundmass consists of augite, hornblende, and felspar. The augite is similar 

 in general character to the porphyritic crystals, but of smaller size, and not quite so 

 perfectly idiornorphic ; it is only in small amount, and is commonly decomposed, while 

 the hornblende remains quite fresh. The hornblende forms almost one-half of the 



