PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 105 



Diabase.] 



3. They may include irregular fragments of other rocks. - 

 III. If they are volcanic: 



1. They may appear to be striped, banded, or pseudo- "stratified" conformably to adjoining sedimentary 

 deposits. 



2. They will probably be accompanied by fragmental (pyroclastic) material, which may or may not 

 itself be really stratified. Such material will vary greatly in coarseness, containing bombs, agglomerates, 

 breccias, tuffs, sands and ashes. The characteristics of these are: (1) Indiscriminate mixture of all sizes and 

 shapes of fragments; (2) Material of same kind as the igneous rocks; (3) Cement, either finer fragmental material 

 (tuff -breccia) or lava (flow-breccia); (4) Very angular shape of smallest fragments (microscopic glass shreds); 

 (5) If ancient volcanoes were on the shore line, such material may have been immediately worked over by water 

 and interbedded with more or less normal aqueous sediments. 



IV. Most important of all, however, is the identification of those characteristic structures known to 

 originate only in glassy, half-glassy, or very fine grained porphyritic rocks, solidifying at the surface, or in very 

 narrow dikes where solidification has been rapid. These will be found to be very persistent and can usually be 

 identified under the microscope in spite of devitrification, alteration or even a considerable degree of dynamo- 

 metamorphism. The most common of these structures are: 



1. A vesicular, scoriaceous, pumiceous or amygdaloidal structure. 



2. A sharply defined, small porphyritic structure with a glassy, half-glassy or felsitic(cryptocrystalline) base. 



3. A spherulitic structure, due to either large or small lithophysae, holjow spherulites, or compact- 

 spherulites, arranged either irregularly, or in more or less discontinuous bands or layers. 



4. A flow structure, produced either by the elongation of vesicles or the parallel arrangement of constitu- 

 ents or crystallites. It may also be produced by the interlacing of different colored magmas (eutaxitic structure). 



5. Corroded phenocrysts, quartz, with embayments, or skeleton crystals due toTapid or imperfect growth. 



6. Microscopic spherulites, globulites, trichites, crystallites, real or devitrified glass inclusions, quartz 

 with orientated siliceous aureoles, axiolites, etc. 



7. Perlitic structure, wholly or partly devitrified. 



Although some of these structures may occasionally occur in dikes or other igneous rocks which have 

 rapidly solidified beneath the surface they are nevertheless so essentially characteristic of effusive lavas, that, 

 in lack of any evidence to the contrary, they may be regarded as fairly safe guides in establishng the effusive nature 

 of rocks. This evidence is beyond doubt, if such rocks are accompanied, as they generally are, by ash material. 



While a single one of these characteristics may not be sufficient to identify a volcanic occurrence, many, if 

 not all of them, will be found to occur together, and only in rare instances will it be found that some of them, 

 at least, have not survived the vicissitudes of metamorphism. 



No. 7C. DIABASE (with olivine). 



Duluth. From a dike in contact with No. 7A, equivalent to No. 47, roughly in line of bearing with No. G. 



Compare No. 47. 



Ref. Annual Report, ix, pages 12, 18, 19. 



Mey. A dark gray diabasic rock of medium grain, ophitic, considerably decayed, 

 and with the feldspars about the only mineral that is fresh. 



Mic. The structure is distinctly ophitic, and the constituents are plagioclase, 

 augite, magnetite and alteration products. The plagioclase is rather fresh. A series 

 of ten sections of the fehkp<u- measured for greatest equal angle of extinction on 

 opposite sides of the twinning line, gave 29i and 30. According to the " epures " of 

 Michel Levy, this might indicate either a labradoritc (Ab 3 An 4 ) near bytownite, or 

 anorthite. Since, however, an optic axis is also found to pierce the section just 

 outside the area of the microscopic field, the feldspar is certainly near labrador- 

 bytownite. In another section of the same feldspar the bisectrix () appears in 

 the centre of the field, with distinct black cross and shifting hyperbolas. This grain, 

 however, being encroached on all sides by surrounding grains, no crystallographic 

 character was afforded for measuring the angle of the axial plane, except on an 

 imperfect cleavage trace. This gave an extinction angle of 59 to 60, which also 

 indicates, so far as it is of value,' a labradorite, or labrador-bytownite. 



