12 Notes on the Diabase Rocks 



strung upon a thread. In places these trichites cross each other 

 like a net- work. In a few cases I have observed that there are 

 no trichites, but then these dust-like particles are diffused 

 in numbers throughout the basis, which fills in the spaces 

 in the crystalline ground-mass. The slices are in such cases 

 very obscure. In rarer cases the basis contains very many 

 rounded granules (sperulitic bodies) of a darker shade of 

 brown than the basis. In such cases I have observed only 

 a few thorn-like microliths, and none of the black dust so 

 frequent in other examples. 



Perhaps the most numerous components of the ground- 

 mass are very minute prisms of felspar. In some cases these 

 form a perfect net-work, separated by the basis, and enclos- 

 ing the other constituents. In most, if not in all, instances 

 these minute felspars seem to be twinned and to be triclinia 



The next most frequent component of the ground-mass is 

 an ore of iron, either in irregular grains or in distinctly 

 rectangular crystals. I believe these to be in almost if not 

 all instances magnetite. At least such would be the case in 

 the sample chosen for analysis, for in it I failed to obtain 

 any titanic acid by special examinations. Still, titaniferous 

 ores are not probably wholly absent, as I have occasionally 

 observed hexagonal forms suggestive of them. Larger 

 masses of black opaque iron ores fill in what probably repre- 

 sent the sites of former minerals, and there is often also a 

 deposit in the ground-mass of ferric oxide (haematite), as 

 well as ferric hydrate (brown iron ore). Finally, needles 

 and prisms of apatite are very frequent in the ground-mass. 



The porphyrinic minerals are the following : — 



Felspars. — All of these are triclinia Their terminal planes 

 are not, as a rule, well developed, and it is common to see 

 them fractured, and the parts pushed aside. These felspars 

 do not seem to have been the first constituents to crystallise, 

 for they include small crystalline grains of augite, and have 

 also been broken by crystals of the same. It is very com- 

 mon for these felspars to have their crystalline planes marked 

 in the interior of the prisms by rows of rounded, oval, or 

 ragged particles of what seems to have been glassy material. 



The composition of these plagioclase felspars is, in almost 

 all cases, by wide rather than narrow lamellae. They are 

 usually compounded according to the Albite law, sometimes 

 according to the Carlsbad law, and in rare cases I have 

 observed cross lamellae which may be said to be interposed 

 according to the Pericline law. It may be said that the 



