44 
Proceedings of the Royal Irish Academy, 
a number of crystals of augite, similar evidence of some feldspar, and 
in most basaltic augites, olivine. So far as my investigation has yet 
gone, the three minerals most frequently found in hornblende and 
augite crystals are, the feldspars, garnets, and olivine. 
The aluminous hornblendes and augites have all crystallized in the 
midst of a mass of orthoclase or potash feldspar, or of plagioclase or lime 
feldspars, which are mixtures, in various proportions, of albite and 
anorthite, or, in certain cases, in the midst of nephelin. Whether 
the crystals were formed by deposition from water or out of a molten 
mass, they must have carried down with them traces of the feldspar or 
other substance, out of which the crystals separated, in the same way 
that all salts enclose more or less of the impurities which are con- 
tained in the solution from which they have separated. 
"When the foreign substances are in solution, whether in water or 
in igneous fusion, and that the circumstances are favourable for the slow 
formation of crystals, crystalline endomorphs are formed. If, on the 
other hand, the hornblende or augite crystals form in a felspathic mass, 
which is not in a condition to crystallize apart at the moment of the 
separation of the meta-silicates, the latter, in the act of crystallizing, 
impose their crystallizing action on some of the feldspar, giving rise to 
hornblende or augite crystals, containing molecules of feldspar, regularly 
associated with the meta-silicates in the crystallographic constituent 
molecules, but not chemically combined with them. The amount of 
such heteromorphic chemical molecules which can become associated 
in a crystalline molecule is limited, but variable, to a slight extent, 
according to temperature, form of the crystals, relations between the 
crystalline series of the crystallizing body, and of the heteromorphic 
compound, &c. These circumstances account at once for the apparent 
definiteness of composition of many aluminous hornblendes and augites, 
and for the slight variation which is often noticeable in the amount of 
alumina in crystals from the same locality. 
When hornblende and augite crystals are formed in the midst of a 
very complex mass, the crystallographic molecules may contain two or 
more kinds of heteromorphic chemical molecules. Generally, how- 
ever, a feldspar, either of the orthoclase group or of the plagioclase group, 
is the only, or the principal, heteromorphic chemical molecule present 
in the constituent crystalline molecule of the augite and hornblende 
crystals. In some instances, a silicate of the composition of garnet is 
also present in hornblende, and of olivine in augites; and in those 
cases, crystals containing these minerals as endomorphs, are often found 
in the locality. I have frequently found in basic highly aluminous 
hornblendes, in addition to orthoclase, an aluminate of the composition 
of spinel Mg^AF'^'gOi. Indeed, it is probable that, in all cases where 
the silica of aluminous hornblende is low, this aluminate is present, 
although I have not noticed it as a distinct endomorph, nor, so far as 
I know, has any one else. 
Where the foreign ingredients are unequally distributed, or where, 
though regularly distributed, they are still distinctly recognizable by 
