450 Prof . Hull — Structure of Porphyry. 



presence of this mineral ; and on the Maps of the Geological Survey- 

 it is named and coloured as " Greenstone Porphyry." But the thin 

 translucent slices, one taken from Lambay Island, and the other from 

 Portraine, fail under the microscope to reveal this mineral ; while, 

 on the other hand, they show that the dark colouring of the base is 

 due to its being highly charged with minute crystalline grains of 

 magnetite, together with which is a little chlorite, imparting the 

 greenish tiuge to the rock. 



While, therefore, from the character of the rock, it is by no means 

 improbable that hornblende occurs as an accessory in some places, 

 yet on the whole it ought to be regarded as a felstone porphyry ; 

 or, to adopt Naumann's term for the quartzless varieties of porphyritic 

 felstones, " a porphyrite." ^ We shall now proceed to consider its 

 microscopic structure in detail, beginning with the base. 



The Base. — When a thin slice is examined with a moderate power 

 (magnifying from 24 to 100 diameters), the base is seen to consist 

 of a colourless felsitic material, amorphous, but sometimes containing 

 long minute felspar crystals. Throughout this base small black crys- 

 talline grains of magnetite are distributed in immense numbers, often 

 so small as not to come into sight till viewed with a high power. 

 Along with the magnetite, a mineral which I take to be chlorite is 

 abundantly diffused in the form of light green clouds, or else filling 

 up along with calcite cavities and minute fissures. The dense and 

 dark appearance of the base is therefore evidently due to its being 

 charged so fully with magnetite, while the greenish tinge is due to 

 the green structureless mineral which is more or less generally dis- ■ 

 tributed throughout the felsitic base ; and which, in all probability, 

 is chlorite. 



Orthoclase crystals. — These are abundant, and sometimes well 

 formed. They frequently occur in pairs, sometimes in threes, less fre- 

 quently in the form of a cross. They vary in size from half an inch 

 downwards. In colour they are pale green, mottled white, and under 

 the microscope are seen to be traversed by cleavage planes and fissures 

 in various directions, altogether different from the fine parallel lines 

 and bands which characterize the triclinic group of felspars. Occa- 

 sionally, however, a banded structure parallel to the sides of the 

 prism is observable in some of the crystals displayed on the polished 

 block in the Gallery of the College of Science. 



Magnetite, — Small black grains of magnetic iron-ore are so abun- 

 dantly distributed throughout the base as almost to obscure it, unless 

 the slice is exceedingly thin. Along with these are also occasionally 

 larger perfect cubes, showing well-defined angles of 90° as estimated 

 by the eye. It is remarkable, however, that it is only at rare inter- 

 vals that a crystal of magnetite becomes enveloped in one of the fel- 

 spar crystals, from which it may be inferred that these latter were 

 consolidated while the iron , remained dissolved throughout the 

 materials from which the base was ultimately formed. 



Very singular, however, is the manner in which these magnetite 

 grains are arranged around the interior of some small cells, as shown 

 2 B. von Cotta's "Lithology," English Version, pp. 168-9. 



