Electron microprobe analyses are presented of chevkinite-group minerals from Canada, USA, Guatemala, Norway, Scotland, Italy and India. The host rocks are metacarbonates, alkaline and subalkaline granitoids, quartz-bearing pegmatites, carbonatite and an inferred K-rich tuff. The analyses extend slightly the range of compositions in the chevkinite group, e.g. the most MgO-rich phases yet recorded, and we report two further examples where La is the dominant cation in the A site. Patchily- zoned crystals from Virginia and Guatemala contain both perrierite and chevkinite compositions. The new and published analyses are used to review compositional variation in minerals of the perrierite subgroup, which can form in a wide range of host rock compositions and over a substantial pressure- temperature range. The dominant substitutions in the various cation sites and a generalized substitution scheme are described.

The distribution of Ascaris lumbricoides within a community was examined at an initial mass anthelmintic treatment programme (January 1984) and following an 11-month period of reinfection (November 1984). Similar patterns of the negative binomial parameter k (an inverse measure of parasite aggregation) and the proportion of parasites within the most heavily infected quartile of the community were recorded at the two dates. The pattern of parasite aggregation within individuals, measured by parameter k, appears to be a stable characteristic of this host-parasite relationship. Significant variation in the intensity of infection was observed between households in the community. The number of family members living in the house strongly influenced the mean Ascaris burden and proportion of relatively heavy infections within adults and children. This finding suggests that the density of people in a house positively influences the frequency of exposure to infective stages of Ascaris, which in turn plays a major role in determining which individuals will harbour heavy infections. Positive correlations were recorded between the initial and reinfection burdens of individuals, relative to others in the community. The correlations were strongest in the youngest and oldest age groups and were more frequently significant among age-stratified groups of females, compared to males. A comparative examination of hypothetical treatment strategies suggests that for Ascaris infections in this community, targetting age groups with anthelmintic treatment would probably be more cost-effective in the long term in reducing the abundance of this parasite than selective treatment of individually identified heavy infections.