606 



MR J. D. FALCONER 



Portions of baked shale may be found here and there still adhering to the upper surface 

 of the dolerite. 



Microscopically, a corresponding variation may be traced in the mineral composition. 

 In the lowest layer (l) large olivines, more or less idiomorphic and entirely replaced by 

 calcite, make up rather more than half the rock ; the remainder is composed of small 

 idiomorphic chloritised augites, ragged flakes of biotite, and numerous lath-shaped 

 crystals of basic labradorite, usually turbid and opaque. The interstices are filled with 

 cloudy analcime which here, as in other portions of the rock, may possibly be in part of 

 primary origin. In the coarse dark-green rock (2) overlying this, felspar is almost 

 entirely wanting, and olivine, purple augite, and brown hornblende, with a little 

 biotite, apatite, and iron-ore, are the principal constituents. The rock is, in fact, a very 



Fig. 1. — Diagrammatic section of Castle Craigs picrite. 

 1. Olivine-felspar rock at lower margin ; 2. picrite ; 3. hornblende dolerite ; 4. fine-grained banded dolerite. 



typical hornblende picrite.* (PI. II., figs. 5 and 6.) The felspar when recognisable 

 belongs to bytownite or anorthite, but is usually much decomposed. The olivines are 

 large and frequently idiomorphic ; the augites have also a tendency to idiomorphism, 

 and are usually surrounded by plates of brown hornblende which, when a little 

 interstitial felspar is present, usually shows good outlines. Sometimes the amount of 

 hornblende and augite seems to increase at the expense of the olivine, and large 

 irregular plates of hornblende are distributed throughout the section, usually enclosing 

 some idiomorphic augites pcecilitically. The occasional spotted appearance of the rock 

 is due to the presence of small aggregates of felspar, much decomposed and stained 

 with iron ore. As the felspar becomes more abundant and the rock becomes finer- 

 grained and assumes a pinkish tint (3), idiomorphic augite remains the most important 

 ferro-magnesian constituent. Olivine is much scattered, and hornblende and biotite 

 build small crystals or fringe the augites. The felspars are lath-shaped and belong 

 mostly to labradorite ; they are usually somewhat turbid and decomposed, and are 

 rarely enclosed in the augite in an ophitic manner. Apatite and iron ores are present 

 in small quantity, and interstitial analcime is sometimes abundant. The picrite has, in 

 fact, passed into a hornblende-dolerite. The upper portion of the sill (4), whose striated 

 character is due to the alternation of more and less felspathic bands, is merely a fine- 



* Cf. Quart. Journ. Geol. Soc, xxxvii. p. 137 ; xxxix., p. 254. 



