266 Scientific Proceedings, Royal Dublin Society. 



some instances a crystal of zircon may be observed half immersed 

 in biotite, and half projecting from it. These facts point to a 

 partial solution of the biotite after its first formation. 



Zonal Felspar. — One of the most remarkable features of the rock 

 is to be found in the beautiful zonal structure of the plagioclase fel- 

 spar. The central mass or core of each crystal consists of a calcareous 

 plagioclase, such as labradorite or andesine, though anorthite may 

 sometimes be present in sharply-defined grains. Surrounding the 

 core follow successive zones or envelopes of other felspars, varying, 

 in composition, but becoming progressively poorer in lime and 

 richer in soda, till in the outermost zone even albite itself may be 

 reached. The core has frequently a dusty appearance, owing to 

 the presence of opaque granular matter, but the envelopes are 

 usually clear and transparent. Muscovite is frequently richly de- 

 veloped as small crystal plates lying along the planes of cleavage. 

 It is here, indeed, next to the margins of the biotite, that this 

 constituent is chiefly collected. The species of epidote before 

 alluded to also occurs in the felspar core, and contributes no small 

 share in some instances to its bulk. Repeated twinning, usually 

 so characteristic of plagioclase felspar, certainly occurs, but com- 

 paratively rarely. 



The zonal felspar is idiomorphic, and thus stands in striking 

 contrast to the microcline. Certain idiomorphic non-zonal felspar 

 crystals I leave to my completed account for detailed description. 



(Juartz. — This occurs in allotriotropic grains, which, as they 

 precisely resemble those of ordinary granites, do not call for special 

 description. 



Microcline. — This, like the quartz, is allotriomorphic, filling up 

 the spaces between the other constituents ; while, however, a plate 

 of quartz may consist of several crystal grains, all differently 

 orientated; a plate of microcline, on the other hand, however 

 irregular in form, and even when nearly cut in two by other closely 

 adjacent minerals, yet retains the same optical orientation over its 

 whole extent, thus recalling "ophitic" structure. In some cases 

 it extends between closely approximated granules of quartz in 

 narrow irregularly-bounded strips, and then appears to have con- 

 solidated later than the quartz. 



Summary of Events. — Zircon and apatite were the first consti- 

 tuents to separate out from the original homogeneous glass, apatite 



