I’m reading (or re-reading,

As a provider of credit, the bank works with all the capital at its disposal; its own and that of others. Its gross profit consists of interest paid on the capital which it has lent. Its net profit – after deduction of expenses – is the difference between the interest paid to it and the interest, which it pays on deposits. This profit is not, therefore, profit in the strict sense, and its level is not determined by the average rate of profit. Like that of any other money capitalist it arises from interest. The position of middleman that the bank occupies in credit circulation enables it to profit not only from its own capital, like any other money capitalist, but also from that of its creditors to whom it pays a lower interest than it demands from its debtors.(…) The level of interest depends upon supply and demand of loan capital in general, of which bank capital is only a part. This level of interest determines the gross profit. In order to attract the greatest possible amount of money for their use, the banks in turn pay interest on their deposits; and the amount of such capital at the disposal of any bank depends, ceteris paribus, upon the level of interest which it pays on deposits. Competition for deposits compels the banks to pay the highest possible rate of interest. The difference between the interest which the banks receive as creditors and the interest which they pay as debtors constitutes their net profit. (p. 172)