Such, in brief, had been the progress of research when the present survey was begun, in 1936. The main objects of this survey were to determine the age and origin of the caldera. To accomplish these ends, it seemed desirable to reconnoiter a wide area surrounding the park in order to appreciate its geological setting with respect to neighboring volcanoes and to discover the nature of the basement beneath Crater Lake. Work begun ten years earlier among the volcanoes of the Lassen Peak region and on Mount Shasta was therefore continued northward into Oregon to connect with the survey of Mounts Thielsen and Bailey made in 1932. In this way a general picture was obtained of the structure and volcanic sequence of the southern Cascades over a distance of more than 100 miles. It seemed desirable also to examine other calderas for the light they might throw on the mode of formation of Crater Lake. With this in mind, a study was made of the large volcanic depressions of New Zealand, of the Javanese calderas, including Krakatau, Tengger, and Idjen, and of several calderas in Japan, particularly those of Aso, Towada, and Toya. The question of how Crater Lake was formed was then approached with greater confidence.
The present report deals chiefly with the results of field work and only secondarily with the microscopic petrography of the rocks, a subject of which Patton has already presented an admirable account in his contribution to Diller’s monograph.
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