68 REPORT — 1856. 



Lastly, in order to obviate the objection that the coal-formation itself might have 

 thinned out in the same direction as the superincumbent formations, Mr. Hull endea- 

 voured to show that the manner of its formation, and that of the secondary strata, 

 were altogether different ; for while (as had been shown by Mr. Godwin- Austen) the 

 ancient coal basin included the greater part of the British Isles, France and Belgium 

 forming one almost uninterrupted coal-growth ; on the other hand, the development 

 of the new red sandstone and lias proved that they are formed of sediment derived 

 from norlh-westerlij sources, and that consequently, as the distance from these sources 

 increased, the quantity of sediment diminished. Hence it was argued, that while under 

 Oxfordshire the strata between the great oolite and the coal-measures might be very 

 thin, the thickness and quality of the coal-seams would not necessarily have deterio- 

 rated. 



On the Alteration of Clay-slate and Gritstone into Mica-schist and Gneiss by 

 the Granite of Wicklow, <^c. By J. Beete Jukes, M.A., F.R.S. 

 The granite of the south-east of Ireland, extending from Dublm Bay into the county 

 of Kilkenny, is intrusive as regards the Lower Silurian rocks, and sends veins into them. 

 The Lower Silurian rocks generally are composed of dull earthy slates interstratified 

 ■with fine-grained gritstones commonly not more Uian an inch in thickness, but some- 

 times two or three feet. The main granite range is not a true geological axis, as it 

 does not bring up the lowest beds of the district, and forms only a partial geographical 

 axis as it is breached through by the valley of the Slaney. 



Wherever granite appears at the surface, it metamorphoses the surrounding slaty 

 rocks and changes them into schistose rocks (mica-schist, &c., and gneiss). 



The dull earthy slates are found on approaching the granite to acquire a " glaze " 

 or silvery lustre not only externally but internally, as it is as apparent in the rock when 

 ground to powder or triturated into mud or silt as in the mass of the rock. This 

 micaceous lustre increases as we approach the granite, till within half a mile (more or 

 less) of its general boundary nothing can be found but schistose rocks, often containing 

 crystals of garnet, andalusite, staurolite, schorl, &c. Simultaneously with this change 

 in mineral structure the rocks are affected by a folding or corrugation, crumpling both 

 slates and grits, evidently the result of a mechanical force. The foliation of the mica- 

 schist is most usually parallel to the original stratification of the rock, as shown by 

 these grit-bands. 



In the cases observed where the foliation crossed the beds, and ran parallel to the 

 cleavage, the plates of mica were smaller and more interrupted than when parallel to 

 the bedding, their development being apparently interfered with by the changes of 

 texture in the original lamination of stratification. 



The surface boundary of the granite is very undulating and irregular, and many 

 large patches of schistose rock are found within it, resting on, and apparently dipping 

 down into the granite. The original surface of the granite appears to have had rather a 

 gentle general slope, but to have been very uneven, having many hollows and 

 protuberances, 



^v Although the lowest beds of the Silurian rocks are not brought up by the granite, 

 yet the beds near it dip every way from it at angles not often exceeding 30°, and the 

 patches of schistose rock lying within the general boundary of the granite dip towards 

 that boundary. The graphite is probably continued under the adjacent slates with a 

 similarly gentle slope and irregular surface ; especially on the eastern side, where many 

 smaller bosses appear at the surface between the main range and the sea. These 

 smaller bosses produce alteration in the slates through which they appear, exactly 

 similar to that of the main range, though of proportionately less extent. 



At Polmounty near New Ross, thick beds of grit interstratified with shale were 

 observed converted into alternations of fine-grained gneiss and mica-schist, and near 

 Graiguenamanagh a dark gneiss was seen, in which crystals of common felspar as large 

 as the thumb were imbedded, forming a true porphyritic gneiss, which is yet nothing 

 more than an altered Silurian gritstone or an arenaceous slate rock. 



The very general occurrence of mica in these schistose rocks results probably from 

 the varied mineral composition of different well-characterized micas, so that true mica 

 (of some kind or other) is more likely to be produced than any other mineral. Inde- 



