The arrest at Baltimore of the secession members oí tho Legislature of Maryland has exploded a piot of the rebels rhiuh will provo of vast importanco to tho government. It is known that eome of tho arrosted secossion members of the Legislature had a programmo all prepared, by which an act wastobo passcd dcclaring Maryland out of the Union and a State in tho Southern Confuderacy. Tho movcment of tho rebel army up tho rivor was preparatory to a contemplnted moveinent across Tho attempt was to be made on the day the Legislatnro aoted. At the same timo tho Mayor of Baltimore, who was also arrested to-day, was to co-operato by tho best demonstration he could mako. By tho arrest of the leaders of this conspiracy tho Legislaturo is without the power to act, anefthe rebels of Baltimore havo lost their chief. A eimultanoous movement of rebela was to tako place along the castern shorc of Maryland, undor tho supposed direction and leadership of ex-Commaoder Buchanan, of tho navy, to aid the rubel army of Virginia in crossing to Maryland. This movcment under Buchanan was tj be made to distract our centre. , Tho plot is exploded by the prompt action of General McClullan, in directingthearrests mado m Baltunoro to-day.