ANKIVEESAET ADDRESS OF THE PEESIDBNT. I3I 



overlie the bedded basalts of the lower part of the volcanic group, 

 but these are the only localities in Scotland, so far as I know, 

 where such rocks are associated with the puy-type of vent. 



ii. Intrusive SJieets, Weeks, and Dykes. — As a rule, rocks which 

 occur intrusively, especially where they form a considerable mass, 

 have assumed a much more coarsely crystalline texture than those 

 of similar chemical composition which have been poured out at the 

 surface. But with this obvious distinction, the two groups have so 

 much in common, that the geologist who passes from the study of 

 the subterranean phenomena of the Plateaux to that of the corre- 

 sponding phenomena of the Puys is at once impressed with the close 

 relationship between the material which, in the case of the puys, 

 has consolidated above ground, and that which has been injected 

 below. There is no such contrast between them, for example, as that 

 between the basic and intermediate lavas of the plateaux and the acid 

 intrusions associated with them. The following chief types of rocks 

 may be recognized among the intrusive masses of the puy-series. 



(a) Olivine- Atjgite E-ocks, Picrite^ 6fG. — The now well-known ex- 

 ample of picrite in Inchcolm, in the Pirth of Forth, occurs as an 

 intrusive sheet among the Lower Carboniferous Sandstones.^ 



Rocks approaching limburgite occur among the sills and bosses 

 which pierce the Carboniferous Limestone series of Fife between 

 Cowdenbeath and Inverkeithing. One of these is found at Pitan- 

 drew, near Fordel Castle. Dr. Hatch finds it to consist of " numerous 

 porphyritic crystals of olivine with a few grains of augite and 

 an occasional small lath-shaped crystal of felspar imbedded in a 

 groundmass which is composed principally of idiomorphic augite 

 microlites, small crystals of a brown mica, granules of magnetite 

 and prisms of apatite. In addition, there is a considerable amount 

 of interstitial matter, which is partly colourless glass, and partly 

 shows a slight reaction between crossed nicols." Another example 

 of the same type of rock occurs as a plug or boss in the tuff-vent 

 of the HiU of Beath. 



(6) Hocks of the character of Olivine-basalt and Olivine-dolekite 

 play a leading part among the sills connected with the puy- 

 eruptions. The presence or absence of olivine, however, appears to 

 be a mere accident of cooling or otherwise, and hardly to deserve to 

 be made the basis of a definite classification. I have shown that 

 in the same mass of rock at Blackburn a gradation can be traced 



1 Trans. Roy. Soc. Edin. vol. xxix. (1879) p. 506. Teall, ' British Petro- 

 graphy,' p. 94. 



