“The orthoclase occasionally presents the adular variety, but this never becomes a marked feature. I have observed the same in many of the trachytes of the High Plateaus and of the Great Basin. Another phenomenon is the occurrence of crystals which are quite typically monoclinic at one end (orthoclase), and at the other end have the arrangement of plagioclase. This is well known elsewhere, and described by Zirkel. (Mik. Beschaff der Mineralien u. Gesteine). “In classifying these rocks therefore, we may observe that they present a blending of the characteristics which are common to trachyte and felsitic porphyry. Those who regard porphyry as a distinct class of eruptive rocks would have no hesitation in calling Nos. 18, 56, 61, 68, and 69 undebatable felsitic porphyries, and to the same series might with propriety be added No. 33. With equal confidence Nos. 16, 20, 35, and 43 may be called unqualified trachytes. The other rocks are intermediate in character between these two extremes, and the whole may be regarded as a series in which the individuals form a graduated scale. “You will recall the fact that many lithologists object to the terms porphyry or porphyritic being used to designate a distinct class or group of rocks, holding that they merely characterize a single feature which is more or less frequently presented by all igneous rocks and having no necessary relation to any of them, and that this is no more adequate to such an important distinction than the color or relative degree of fineness or coarseness of texture. Although this latter view seems to me to underrate the distinctive value of the porphyritic character in general, I incline very decidedly to the belief that it is true as applied to these Henry Mountain rocks. Here at least the porphyritic character has but little significance. In some varieties the crystals are larger and more perfect and the groundmass more homogeneous; in others the crystals are smaller and imperfect, and the groundmass more coarse and irregular; while still others are ‘betwixt and between’. The most careful scrutiny fails to show any fundamental differences in the groundmass or in the included minerals. Hence I think these rocks would be accurately designated _as a group_ by calling them porphyritic trachytes. Such varieties as Nos. 16, 20, and 35 may by themselves be called simply trachyte, the porphyritic character being insufficiently distinct in them to warrant any qualification of the name.