Alexander Scott — The Crawfordjohn Essexite. 517 



of dusty cryptocrystalline material and partly of clear grains of 

 felspar. Under a low-power objective the latter appear as small 

 rhomb-shaped crystals, but a high power shows that they are 

 irregular in sbape and have no definite crystal boundaries, merging 

 gradually into tbe dusty material. The felspar 6eems to be an acid 

 oligoclase as the refractive index is sometimes below that of Canada 

 balsam and sometimes above. This rock has some resemblance to 

 a fine-grained adinole, and only differs from the latter in tbe presence 

 of some anorthite in the felspar. 1 A portion of the soda in the 

 mineral has probably been introduced from the intrusive rocks and 

 the fact that oligoclase is present, instead of albite as in the typical 

 adinoles, may possibly be ascribed to the presence of calcium oxide in 

 the original sediments. One fact in favour of the idea that the 

 intrusive rock is the origin of the soda is the existence of numerous 

 veins of adinole much coarser iu grain than the rest of the rock. 

 These veins consist of a core of muscovite, sometimes replaced by 

 decomposition products and flanked by bands of clear felspar. In 

 other cases, the relative disposition of the mica and felspar is quite 

 irregular. The veins often cut across the bedding planes in an 

 irregular fashion, but occasionally they lie along the latter. This is 

 particularly the case along the junction of two different types of 

 sediment, which are usually found separated by a band of fairly 

 coarse adinole. 



In the disused quarry, several types of rock, whose original nature 

 is doubtful, are found. One type, which in the hand-specimen shows 

 faint spherulites, is seen under the microscope to consist of numerous 

 microcrystalline patches set in a cryptocrystalline matrix. The 

 former consist of groups of clear felspar, permeated with dark 

 material, which under a high-power objective can be resolved into 

 aggregates of greenish microlites apparently of a pyroxenic nature. 

 Dark elongated masses of similar material are scattered throughout 

 the groundmass and appear to represent former crystals of mica or 

 hornblende. Occasionally new-formed ilmenite is found. Sometimes 

 clots and bands of a rather different nature are found enclosed in this 

 rock. The ferromagnesian areas are more rounded, while the felspar, 

 which occurs as small laths, is uniformly distributed throughout 

 a cryptocrystalline groundmass. The felspars appear to be oligoclase, 

 as they have a low extinction angle and a refractive index above that 

 of Canada balsam. Parts of the groundmass have a lower refractive 

 index, and may be orthoclase. 



As the intrusion is approached the felspathic rock assumes a more 

 decidedly porphyritic aspect. The felspars, which show broad 

 rectangular sections, are sometimes arranged in groups and sometimes 

 found as single crystals. The mineral is perfectly colourless and fresh 

 and sharply delineated from the groundmass. Carlsbad twinning is 

 universal and albite twinning fairly common, while occasional 

 striations following the pericline law can be observed. The refractive 

 index and extinction angles indicate a composition approaching 

 andesine. Occasionally phenocrysts of orthoclase are also found. 



1 Cf. J. Both, Chemische Geologie, vol. iii, 1893, pp. 141-4 ; H. Dewey & 

 J. S. Flett, Geol. Mag. [5], vol. viii, pp. 243-4, 1911. 



