Differentiation trends in rock bodies of various sizes and types have been studied, from different viewpoints and with contrasting results, by many geologists in this country and abroad* Such studies are of major importance, for magmatic segregation has been proposed by several authors as a means of generating various rock sequences (Bowen, 1956; Wager and Mitchell, 1951; Walker, 1940). Bowen has proposed that most of the rock types occurring at tdie earth's surface today may have been produced chiefly by fractional crystallization of a primary magma, and other authors (among others, Walker, 1953; Edwards, 1942) have demonstrated that rock melts are differentiated into phases of different compositions and textures during crystallization. No detailed work has been published on the differentiation of lamprophyric rocks, which though of minor abundance, are widespread in granitic terranes. It was therefore considered of value to sample and study a well defined lamprophyre intrusion in the hope that definite differentiation trends might be discerned and that, thereby, some information regarding the genesis of this little understood rock type might be gained. The sill chosen for study is located in the La Plata Mountains, southwestern Colorado, in the northeastern part of the La Plata quadrangle (Figure 1). The sill is sufficiently well exposed that it may easily be sampled from upper to lower contact. The contacts are well defined, and there has been little contamination by the wall rock. The sill has evidently been only slightly deformed since the magma was intruded, the only evidence of disturbance being a fault of about thirty feet displacement near the center of the sill. No detailed petrography of the intrusion had been undertaken previously, The choice of this particular body for the present study was made on the basis of its good exposure and on a general field and mineralogical description given by Cross (1899), who indicated an apparent layering within the sill. The field work consisted of sampling the sill vertically from upper contact to lower contact at intervals of one to five feet. A total of thirty-three hand specimens was thus collected, and thin sections of these samples were then examined petrographically, Modal analyses of each section were made by point-counting 1,000 to 1,300 points. Detailed optical properties of the pyroxenes and feldspars were determined with the aid of a Zeiss four-axis universal stage. This thesis is concerned chiefly with the microscopic analysis of the rocks collected in the field and the results derived from this analysis. A detailed petrographic description of the rocks is given* and some conclusions regarding the manner of differentiation of the sill are reached. A possible mode of genesis of the lamprophyre magma, based on results of this investigation, is postulated.