Highly altered igneous rocks known in the Middle Valley, the Juan de Fuca Ridge were recovered in Hole 856H during ODP Leg 169 (Bent Hill Massive Sulfide). Alteration of basalts and dolerites from Hole 856H, that penetrated to a depth of 500 meters to the upper part of the oceanic crust in an area of extreme hydrothermal activity, was under greenschist facies in conditions of a discharge of hot solutions with temperature approximately 300-350 degrees C. The principal secondary mineral in the igneous rocks is Mg-Fe chlorite, albite, amphibole, epidote, and quartz are found in lesser amounts. Almost all chemical elements are mobile at hydrothermal alteration of the igneous rocks under the influence of hot solutions on the rocks. In these alteration conditions the igneous rocks are depleted in Ca, Na, K, Cu, Zn, Rb, Sr, and S (sub Total) , Si, and Al; Locally, they are depleted in Cr, Ni, Eu, Ce, Nd, La, and Nb. Altered rocks record that alteration leads to an increase in Ti, Fe (sub Total) , Mn, and Mg and locally, to enrichment in P, Tb, Yb, and Lu in altered rocks. The altered rocks are chracterized by elevated or reduced contents of Co, V, Y. Zr is an immobile element for all groups of altered rocks in this environment.