It is interesting to compare the above weight percent image in the first post, in which the color scale was adjusted manually, seen here again:
With this image below, which are the same data but just displayed using the automatic log scaling with no adjustment. The only downside I can see to the log scaling is that you have to calculate the actual weight percents from the log values in one's head!
For more information on point analyses of trace elements in quartz, see this post:
http://probesoftware.com/smf/index.php?topic=29.msg387#msg387