42 Proceedings of the Royal Irish Academy. 
in these tlu'ee questions, which have been answered positively by Herr 
Tscherniak. 
The point not embraced in the three questions, just stated, is — whe- 
ther the aluminous compound, or compounds, are present asheteromorphic 
ingredients in a state of mixture, or in a state of chemical combination, 
and may be discussed first. If all the constituents were combined into 
one compound, the crystallographic molecules, which such a compound 
may form, should be homogeneous, and, with few exceptions, perfectly 
similar; and hence the crystals should not exhibit the anomalous 
cleavage which is so characteristic of heteromorphic crystals. It is 
probable, therefore, that the dyadic meta-silicates are not combined 
with the aluminous compound or compounds. Foreign ingredients 
may be present in crystals in three ways : as distinctly crystallized 
endomorphs, recognizable by the naked eye, or by the aid of the mi- 
croscope ; as regularly or irregularly distributed impurities, not dis- 
tinctly crystallized; and as uniformly distributed heteromorphic 
materials, not distinguishable by the eye or microscope, but the pre- 
sence of which may be recognized by variations in the angles, the 
character of the cleavage and the faces, the inequality of weathering, 
&c. 
The study of the endomorphs, or crystals of foreign substances en- 
closed in other crystals, is the first step towards answering the three 
questions put above. Such crystals indicate, if they do not absolutely 
tell, the nature of the foreign ingredients, which may be present in 
such a state as to be unrecognisable by the ordinary means. Under 
the term endomorph, we may also include the fine films which insinuate 
themselves between the cleavage planes, or fill up the fine fissures or 
cracks, which may be detected in many crystals by the microscope. 
The materials which fill such fissures, &c., is generally derived from 
what may be called the mother-liquor, out of which the crystals se- 
parated — whether of aqueous or igneous origin. The only endomorphs 
I need notice here are those containing alumina. I shall, therefore, 
pass over the crystals of many non-aluminous minerals which have 
been detected in hornblende and augite crystals ; such as apatite, the 
frequent occurrence of which, I was one of the first to point out. 
The aluminous endomorphs which are found most generally in 
hornblende crystals, are — feldspars of the orthoclase varieties, nephelin, 
cyanite, olivin, garnet and idocrase, mica and talc. Those generally 
found in augite are, feldspar, chiefly of the plagioclase variety, 
especially Labrador, but also orthoclase, nephelin, leucite, garnet. I 
shall mention a few examples of the occurrence of these endomorphs, 
by previous writers. Delesse"^' has described hornblende crystals from 
veins in the gneiss of St. Phillip, in the Vosges, which are obliquely 
intersected by microscopical veins of a feldspar, having a fatty glance. 
According to Sandberger,f needles of sanidin are frequently enclosed 
in hornblende crystals in the trachytes of T^assau. In the black opaque 
* Annales des Mines, [4] xx. 165. f Poggend.^ Ann. Ixxxiii, 455. 
