SINCE THE INEQUALITIES in income are large in this country, it's not surprising that the inequalities in families' accumulated wealth are even larger. The results of the Census Bureau's recent survey make that point dramatically. But people's wealth is extraordinarily difficult -- perhaps even impossible -- to measure accurately, and the Census figures need to be read with caution. It's a good guess that the poor are even less well-off in relation to the rest of the country than the Census report suggests, but that the broad middle class is better off.

More is known, and known more precisely, about the distribution of American wealth in 1776 than today. In the 18th century wealth was mainly real estate and tangible objects, such as jewelry, that people listed in great detail in their wills. If you are curious about the disparities between rich and poor in Revolutionary America, you will find the best kind of evidence in the records of estates. But they won't show you much about the structure of wealth today. One reason is that, because of inheritance taxes, the truly rich often go to great lengths to pass wealth along to the next generation before they die. Another and more important reason is the difficulty in estimating the value of assets that expire with the owner -- pensions and insurance coverage. That value at any stage in a person's life depends on a lot of unpredictable things, such as life span and the inflation rate. There's no market price, for a person can't sell his pension rights. The Census Bureau, wisely, leaves pensions and insurance out of this survey altogether. But they are a very substantial part of middle-class wealth.

To the extent that it's reliable, the survey confirms to a startling degree a pattern that's already familiar. The average family's wealth, meaning its net worth, is not much more than one year's income. But you already knew that the United States is a nation of spenders, not savers. The median white household's wealth, at a modest $39,135, was 12 times that of the median black household. A much larger proportion of blacks than whites are poor, and poor people accumulate very little wealth.

It will be interesting to see what a comparable survey of wealth shows a few years into the future. For some time it has been clear that the disparities in income are widening in this country. The process has accelerated in the Reagan years, but it seems to have started much earlier, in the latter 1960s. No one has yet come up with an entirely satisfactory explanation, but the trend is beyond question. The report on wealth is a snapshot, showing great distances among rich, middle class and poor. The income data, more reliable, show that those differences are growing. American society is changing, and, in this respect, the change is not for the better.