. Bulletin of the British Museum (Natural History), Mineralogy. i78 PETROGRAPHY AND PROVENANCE graphic character. This is shown in qualitative terms in Table I. The distances used as a basis for definition of " local ", " remote ", etc. are those which give the closest correlation with petrographic groups. Table I shows that the order of increasing proximity to source is, in terms of main petrographic groups, I, II, IV, III. It can be demonstrated that it is also the order of decreasing utility as hone- stones, assuming that this is related to measurable factors of texture
. Bulletin of the British Museum (Natural History), Mineralogy. i78 PETROGRAPHY AND PROVENANCE graphic character. This is shown in qualitative terms in Table I. The distances used as a basis for definition of " local ", " remote ", etc. are those which give the closest correlation with petrographic groups. Table I shows that the order of increasing proximity to source is, in terms of main petrographic groups, I, II, IV, III. It can be demonstrated that it is also the order of decreasing utility as hone- stones, assuming that this is related to measurable factors of texture and composition. Table I Distance relation of Approximate Petrograph ic group probable provenance distance _A_ r â¢Â» to site (miles) I II III IV (a) Local 250 X X 0 0 X Dominant probable sources (for group IV, (a) and (b) are of sub-equal importance). x Subordinate probable sources, o Improbable as sources. All honestones consist of angular or subangular grains of minerals harder than the relevant metal, in this case iron or steel (hardness usually exceeding 6 on Moh's scale, generally quartz, H = 7) embedded in a matrix of minerals softer than steel (H = 2 to 4: micas, chlorites, calcite, limonite, etc.). They vary as to the grain-size and the content of the hard components, both of which can be easily estimated. To study this variation, all the honestones of which thin sections of adequate size were avail- able (about one in three of the total number) were measured. Grain-size was estimated in all cases, on the basis of the maximum apparent diameter of 100 grains selected by a randomizing procedure; in cases of a tendency to general tabular elonga- tion, the measurement was made normal to this, but such cases are few. The proportion of hard components was measured by the Rosiwal method, using a Leitz integrating micrometer stage; in this case the number of hones measured was smaller, as very fine-grained rocks (grain-size below 30 fi or so) were omitted as being unsuited
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