A THEORY OF BULBOUS BUDDING 647 



lished) or by a notable rise in the temperature of the supply, as will 

 appear in the following sections. 



3. BULBOUS BUDDING 



With suitable temperature the numerous small flows will form bulbous 

 or elongated masses. On exposure to the air a tough membrane quickly 

 forms on the surface of each little outburst, and the pressure of the lava 

 within expands each bulb by stretching the skin until stopped by its in- 

 creasing rigidity. The stretching produces in the thickening and stiffen- 

 ing crust a pseudo-flow structure parallel to the outer surface by the 

 stretching and flattening of vesicles and the tangential arrangement of 

 microlites and other individualized or segregated constituents of the 

 magma. 



On final stiffening of the crust, with continued pressure of the liquid 

 contents within, cracking occurs and the process of budding is repeated, 

 and so on indefinitely as long as the supply of lava continues in suitable 

 amount and at favorable temperatures, the liquid flowing through a suc- 

 cession of pillows connected by short necks or necks of no appreciable 

 length at all. Some of these channels may become enlarged by the sol- 

 vent action of the magma if the temperature of the supply is sufficient, 

 or all of them may be closed by freezing as the fiow through them dimin- 

 ishes or the temperature falls. This latter event may be postponed by 

 layers of pillows of succeeding generations that form on the first, each 

 layer protecting the pillows beneath from rapid cooling and prolonging 

 the period during which lava may continue to pass through them and 

 increase the extent of the flow. 



Thus tlie process, as I have conceived it, resembles very closely that of 

 paJhoehoe, but not "viscous pahoehoe," which does not seem to exist ex- 

 cept as to a relatively thin crust on a fluid lava. In this budding process 

 the individual pulsations are smaller and a restraining crust forms more 

 quickly, thus preventing the tendency to broad, flattened forms that 

 characterizes typical pahoehoe. Probably also the temperature of the 

 lava is somewhat lower in most cases, and this would further hasten the 

 .production of the viscous film, on the prompt development of which the 

 structure depends. Hence with varying conditions all degrees of transi- 

 tion would occur between typical pillow structure and pahoehoe, either 

 passing into the other or alternating with it, according to circumstances. 



I SEMI-MOLDED, IMMOBILE FORMS 



As these masses are formed, first on the ground or the floor of a lake 

 or sea and then in more or less rapidly succeeding generations one on 



