294 PROCEEDINGS OP THE GEOLOGICAL SOCIETY. [Apr. 5, 



volcanic rocks of the plateau. It consists of a red gravelly matrix 

 of dolerite debris, in which, are imbedded angular and subangular 

 fragments of various igneous rocks, sometimes a foot and a half long. 

 Again, at the south end of the island, opposite the rock called 

 Dubh Sgeir (Black Skerry), the dolerites contain a breccia which 

 swells out rapidly from a few inches to 6 or 8 feet in thickness ; it 

 is a rough nodular bed, varying in colour from a dirty green to a 

 dull red, and consisting of rude angular and subangular pieces of 

 various dolerites, but more particularly of that on which it lies, im- 

 bedded in, or wrapped round by, a greenish more or less crystalline 

 paste, veined with calc-spar. 



2. Intrusive Bosses, Sheets, Dykes, and Veins. 



The Oolitic rocks, as well as the basalt-plateau which lies upon 

 them, are pierced by many intrusive masses of igneous rock. These 

 are all crystalline rocks, no example of any intrusive fragmental 

 mass, such as the agglomerate of necks, having yet been noticed. 

 While in the interbedded series the order of superposition furnishes 

 us at the same time with the relative age of the volcanic beds, among 

 the intrusive rocks we have no certain guide to relative antiquity, 

 save the obvious examples where one rock cuts through another. 

 Nor is it easy to discover any means of ascertaining how far the in- 

 trusive masses were coeval with, posterior, or anterior to those of the 

 plateau. The dykes, indeed, must be newer than the interbedded 

 rocks already described ; for they are found cutting through even 

 the highest of the sheets of the plateau, as well as the intrusive 

 sheets near or at the base. There is reason to think that the pitch- 

 stone-veins are yet more recent. But without attempting any 

 chronological arrangement, let me here describe the intrusive rocks 

 of Eigg, in accordance with the nomenclature above proposed, as 

 capable of classification after the character of the mould into Y'hich 

 they have been intruded. 



a. Amorphous Masses or Bosses. 



Only three amorphous intrusive masses were observed by me in 

 Eigg ; but they possess considerable interest, inasmuch as they serve 

 to throw some light upon the age of similar masses in Skye. They 

 consist of felstone (that is, a supersilicated felspar rock, with a 

 little free quartz), and thus stand out strongly marked from the sur- 

 rounding basic basalt-rocks. The largest and most characteristic 

 forms a range of bold cliff, from 150 to 200 feet high, at the extreme 

 north end of the island. It appears to have risen approximately 

 along the bedding of the Oolitic strata, and thus to form of itself a 

 large rude bed. It consists of a pale grey quartziferous porphyry, 

 traversed by horizontal and oblique veins of basalt. It is quite 

 columnar in places ; and as the sea has here and there hollowed out 

 caves at the base of the cHif, the roofs of these recesses expose the 

 truncated ends of the columns. This rock closely resembles some of 

 the finer-grained parts of the quartziferous porphyries of Skye and 



