When agricultural man had discovered a reliable calendar and was able to definitely determine the time for planting, growth, and harvesting of his crops, his life became still more rigidly fixed in sedentary conditions; he no longer was a hunter or shepherd; he ceased to have a nomadic tendency. The consciousness of being able to rely upon a definite food supply expresses itself in the art of building. He is led to construct more durable habitations. Successful agriculture, stable architecture, and a reliable calendar are thus closely connected. The most successful agriculture in aboriginal North America is found in regions where knowledge of the calendar was most highly developed. Early efforts to perfect the calendar by studies of the sun intensified sun worship. The most highly developed expressions of solar worship as well as the best constructed masonry on the American continent are associated with the highest development of the calendar. There can be adduced no better illustration than the masonry of Peruvian temples which compares favorably with any in the world. The surface ornamentation of these buildings is not as elaborate as in those of Central America, but there are few examples of masonry in the Old World with stones more accurately fitted together, the walls more enduring—a remarkable fact when we consider that the people who built these colossal structures in the New World were unfamiliar with the metals, iron and steel. Sun worship is the basis of the ancient Peruvian culture expressed by these extraordinary buildings. Although our knowledge of Peruvian calendric signs is not as accurate as of that of Central America, all evidence goes to show that the calendar of the Incas was not inferior to that of the Mayas. In prehistoric North America we find remains of buildings constructed of masonry quite equal to that of the same epoch in the Old World. This may be illustrated by reference to the cliff-dwellers’ towers in our Southwest. If some of the towers of Sardinia were placed side by side with those of southwestern Colorado, any impartial observer would say that the masonry in the latter was equal to that in the former. The megalithic dolmens of England exhibit no walls superior in masonry to massive walls in the mountain canyons of Utah and Arizona constructed before the advent of the whites. In other words it is evident that the architecture of a people is not wholly an index of stage of culture. If the prehistoric aborigines of our Southwest be judged by buildings we may say they had progressed in historic development into a stage attained by nations more advanced because they were acquainted with metals. The prehistoric people of our Southwest called pueblos and cliff-dwellers constructed many different forms of rooms which can be compared and reduced to a few types. It is the object of the following pages to examine the morphology of these buildings.