Geology of Massachusetts. 37 
square mile in the State. We have two quite distinct varieties. The 
purest, sometimes called pipe clay, is found almost exclusively on 
Martha’s Vineyard. This is white, and contains usually so little iron, 
that when burnt, it becomes still whiter, and will resist a high degree 
of heat. Hence it is employed for making what are called fire 
bricks, which are used for lining furnaces. White pottery is also 
made from it. But the more common clay turns red on burning, in 
consequence of the oxide of iron in it; and this renders it much ea- 
sier to be melted by the heat, and consequently diminishes its value. 
It is of immense value, however, to the State ; because good bricks 
may be made from it; and because it exists so abundantly in almost 
every town. ‘The same tertiary formation that supplies clay so plen- 
tifully, yields an abundance of sand for the mortar by which they 
must be cemented. ‘This sand, however, is generally rather fine; 
and I am inclined to believe, from all that I can learn, that our mor- 
tar is generally prepared from sand that is too fine. 
Substitute for Fuller’s Earth. 
The common clay along the Connecticut river, has recently been 
employed in Northampton, in the place of fuller’s earth, in cleansing 
cloth. A considerable quantity of it has also been sent down the 
river, for use in other places. This clay is fine grained, and when 
dry, adheres strongly to the tongue. It is said to answer exceeding- 
ly well in the place of fuller’s earth ; on this point however, I have 
my information at second hand. A clay of precisely the same char- 
acter has recently been put into my hands from Leominster, where 
it occurs in alternating layers with sand. Some of the sand of this 
tertiary formation, especially in the gneiss region, is of a delicate 
white color, and quite pure. In some cases, when its finest particles 
are mixed with clay, it will answer very well for giving a polish to 
brass and other metals. 
Clay used in the manufacture of Alum. 
The white clay of Martha’s Vineyard, is employed extensively in 
the manufacture of alum, in Salem ; by the process of Chaptal, I sup- 
pose; although the details are, I believe, kept secret. By his metb- 
od, sulphur and nitre are burnt in a chamber with the clay, which, 
after a considerable time, is lixiviated, and the ley evaporated. 
There is indeed, a variety of Clay which contains sulphur, that will 
produce alum without the addition of other materials; but J cannot 
