Mortgage when to be by assurance.—Where the principal money secured is one hundred rupees or upwards, a mortgage other than a mortgage by deposit of title-deeds can be effected only by a registered instrument signed by the mortgagor and attested by at least two witnesses. Where the principal money secured is less than one hundred rupees, a mortgage may be effected either by a registered instrument signed and attested as aforesaid, or (except in the case of a simple mortgage) by delivery of the property.