Wherry discussed the origin of this material and on the basis of mineralogical and structural evidence suggested its derivation from aan

Wherry discussed the origin of this material and on the basis of mineralogical and structural evidence suggested its derivation from aan original sedimentary type. He concluded that the presence of sillimanite indicated a noteworthy alumina content of the original sediment and thought that very likely the material w’as originally an argillaceous sandstone. The present writer” has discussed the serieitization of certain types of the Moravian Heights and has found that the areas of s&cite encroach upon and cut across the stretched and strained quartz crystals shown in the thin sections. This would indicate that the alteration had occurred definitely later than the stress responsible for the oriented crystals. In all probability, we may assume that the original sedimentary material, which was likely a shaly sandstone as interpreted by Wherry, was completely metamorphosed into a quartz-sillimanite schist with, very likely, accompanying mica, before the introduction of the solutions responsible for the serieitization. In its present form the Moravisn Heights commonly shows only scattered remnants of the original sillinmnite needles. In some places the thin sections indicate the former presence of occasional sillimanite prisms only by the occurrence of small square areas now occupied by s&cite scales and fibres. Such square cross-sections of sillimanite partially altered to sericite are found in other thin sections and indicate the true nature of these bodies in the rock. With the invasion of a granitic magma, the Moravian Heights formation was acted upon by at least two major processes: (1) The ’ formation was intimately injected by granitic materials which parted the original sehistic and gneissic planes and introduced granite layers between them. This type of material is that which most commonly occurs in those districts indicated as Moravian Heights. In some of these specimens the volume of introduced granitic material is dominant over that of the prinmry material. (2) At a later period, hydrothermal solutions attacked the Moravian Heights formation and altered the rock in part to sericite. The breaking down of the original minerals resulted in the serieitization of the sillimanite first, with the partial sericitization of quartz, following or accompanying this other change. As observed at present, therefore, the Moravian Heights rocks sre considerably recrystallized and hydrothermally changed. What mas originally an argillaceous ssandstone is now partially sericitized layers and lenses of material often largely dominated by the invading granite nms8. d.‘&e! I