136 Proceedings of the Royal Irish Academy. 



varieties, but only interstratified with those that are surface-formed. 

 Elvanyte, on the other hand, is a hypogene rock, and, as its natural 

 place is near the granite, it should more frequently occur among the 

 metamorphic rocks. This seems to be the case ; for, years ago, while 

 reading different accounts of tracks of metamorphic rocks, but espe- 

 cially Logan's reports on those of Canada, I was struck with the 

 frequent occurrence of elvanyte among them. .They do, however, 

 range into other strata. In Yar-Connaught, although they all occur 

 associated with the metamorphic rocks, yet many of them apparently 

 have no connection with that group, or the Gralway type granite, 

 as for instance those previously mentioned as occurring in the vicinity 

 of Kylemore, south and south-west of Bengooria, also many in the 

 country to the eastward of Clifden. These may be of Sikuian age, as 

 they graduate into eurytes identical with the eurytes interstratified with 

 the rocks of Upper Llandovery age. If this surmise is correct, we have 

 data on which to calciilate the depth beneath the earth's surface at 

 which a felstone or other plutonic rock will graduate into an elvanyte. 

 The mass of euryte at Benchoona is about 2000 feet above the base of 

 the Silurian rocks ; but if we suppose a thickness of the metamorphic 

 rocks equal to half of this was denuded away in the neighbourhood of 

 Ballynahinch lake and Clifden, where these rocks are so numerous, 

 these would be a total thickness of 3000 feet. There is, however, 

 another locality in Ireland, namely, the county Limerick, where an 

 estimate also can be made. In that county there are bedded 

 dolerytes, melaphyres and eurytes in the carboniferous rocks, at the 

 junction of the lower and middle limestone, and at the junction of 

 the limestone and the coal measures ; while in lower rocks, near 

 the base of the carboniferous limestone, are pipes or protrusions of 

 basic elvanyte, evidently the roots of some of the rocks that at 

 higher zones were irrupted among and bedded with the derivated rocks. 

 Between the rocks where these roots occur and the junction of the 

 lower and middle limestone there is a thickness of about 1000 feet, 

 and to the base of the coal measure of about 2300 feet. If, therefore, 

 these roots belong to the igneous rocks of the lower zone, elvanyte 

 may form at a depth of about 1000 feet, while if to the upper, at a 

 depth of 2300 feet. 



It was previously stated, that between the intrusive oligoclasic 

 granite and the metamorphic sedimentary rocks intrusions of the 

 orthoclasic granite were not uncommon. This, if the above sugges- 

 tions are correct, might natiu-ally be looked for. For as the mass of the 

 oligoclasic-granite cooled, it would shrink from the adjoining rocks, 

 thereby causing vacancies into which the non- consolidated orthoclasic 

 granite should be intruded, and also into all the cracks and fissures in 

 the associated gneiss and schist fonned during their cooling, they 

 necessarily having been heated and expanded while in contact with 

 the oligoclasic granite. 



It seems to be a very generally-received opinion that all granites 

 metamoi-phosed the rocks with which they come in contact ; to me, 



