[Easy] margin requirements permitted the investor to enter the market on a shoestring. By buying on margin, the investor had to pay only a fraction of the quoted price of any particular security. The additional money needed to cover the purchase was supplied by the broker, who obtained these funds from a bank with which he had deposited his customer's stock as collateral. The margin buyer was particularly vulnerable to even a small decline in stock quotations. With any decrease in security values he would have to pay the additional money to cover the corresponding decrease in his collateral. If he should be unable to supply this money - and usually he could not - the broker would be compelled to sell the stock to protect himself at the bank. Once this process had started there was always the danger that it could not be stopped. (P)rices would be further depressed, and more margin buyers would be compelled to dump more stocks on the market. The circle would then be complete, for there was no apparent way of checking this downward spiral after it had been set in motion.
- Harry J. Carman and Harold 0. Syrett, A History of the American People, 1952.

1.1) Summarize this document. How does this document relate to the causes of the Great Depression?
1.2) Is this a primary or secondary source?