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Proceedings of the Rot/al Irish Academy. 



or so in thickness, considerable interfusion and intermodification of 

 the materials have taken place. The junctions seen in the upper levels 

 of the small quarries on the peninsula frequently suggest an actual 

 melting of the lower portion of the shale. The curved and mammil- 

 lated under-surface becomes in places not well marked off from the 

 basalt ; and microscopic examination shows that the two rocks have 

 ''run" in one another (fig. 2) The magma that has given us 

 close at hand the well known ophitic dolerite of Portrush has here 

 cooled as a very fine-grained and grey basalt, with porphyritic crystals 

 of olivine and a few clear felspar prisms, insufficient to form a mesh. 

 The ground between these consists of a very delicate felt of felspar, 

 granular pyroxene, and magnetite, with traces of brown mica on an 

 equally minute scale. Prismatic pyroxene develops freely at the 

 actual junction with the altered shale, but seems to belong as much 

 to the igneous rock as to the sedimentary. Then follows the pyrox- 

 enic type of altered Liassic shale. In places the latter has been 

 streaked out, clearly in a viscid state, until it mingles, in a common 

 flow-structure, with its invader. 



In another example, also collected by myself, the basalt displays 

 a distinct mesh of felspar, with granular augite, passing towards an 

 ophitic structure ; but the microscope reveals in its midst patches of 

 undefined outline, which can be nothing else than partially digested 

 fragments of the calcareous shale. The specimen from which the 

 section was cut shows streaky bands of altered shale, wrapped round 

 by dark oli vine-basalt, which is closely commingled with them. 



Occasionally a granular mineral, with striking pleochroism, its 

 axis- colours ranging from pale green to rich brown-red, is found in 

 the contact-zone of the shale. In one of Portlock's specimens, this 

 mineral appears also within the dolerite near the j unction ; but it was 

 developed before the felspar, while the green pyroxene of the same 

 rock arose later than the felspar. From its mode of occurrence in the 

 altered shale, and the occasional inclusions of colourless material in 

 the centre of its granules, I believe this handsomely pleochroic 

 substance to be andalusite. If this be correct, its presence in the 

 dolerite is likely to be due to partial absorption of the shale. 



One of the most interesting features suggested by the contact- 

 rocks of Portrush, and by similar instances, is the accumulation of 

 one mineral, in our case pyroxene, to the exclusion of others, in the 

 actual contact-zone within the invaded rock. It is possible that this 

 is due to a selective absorption of certain materials, those that are, 

 under the special circumstances j more fusible or more misciblc with the 



